Hamyar Energy NFPA 69 - 2002

NFPA 69 Standard on Explosion Prevention Systems 2002 Edition

Copyright © 2002, National Fire Protection Association, All Rights Reserved This edition of NFPA 69, Standard on Explosion Prevention Systems, was Systems, was prepared by the Technical Committee on Explosion Protection Systems, and acted on by NFPA at its May Association Technical Meeting held May 19–23, 2002, in Minneapolis, MN. It was issued by the Standards Council on July 19, 2002, with an effective date of August 8, 2002, and supersedes all previous editions. This edition of NFPA 69 was approved as an American National Standard on July 19, 2002. Origin and Development of NFPA 69

In 1965, an NFPA Committee was appointed to develop standards for explosion protection systems. These standards would include information on inerting to prevent explosions and on venting to minimize damage from an explosion. A tentative draft on explosion prevention systems was presented at the NFPA Annual Meeting in New York City in May 1969. This tentative document was officially adopted in May 1970. NFPA 69 was revised in 1973 and reconfirmed in 1978. In 1982, the Committee on Explosion Protection Systems began a thorough review of NFPA 69, including the development of a chapter on the technique of deflagration pressure containment. The results of that effort became the 1986 edition. The 1992 edition of NFPA 69 incorporated a new chapter, on deflagration isolation systems. Partial amendments were made to refine definitions, improve descriptions of oxidant concentration reduction techniques, improve material on deflagration suppression, and fine-tune deflagration pressure containment material. The 1997 edition of this standard included some reorganization and updating of the technical material to improve its usability. New material was added on enrichment to operate above the upper flammable limit as a means of explosion protection with minimum oxidant concentrations for preventing explosions. Material was added for provisions on reliability of explosion protection control systems and deflagration suppression systems for consistency Copyright NFPA

with other NFPA standards. The 2002 edition of NFPA 69 includes new information on spark detection and extinguishment system design. A reorganization of the protection methods now reflects a hierarchy based upon the degree of explosion prevention. The LOC values for gases and vapors in Annex C have been updated based upon recent research. The standard has been revised to reflect NFPA Manual NFPA Manual of Style requirements. requirements. Technical Committee on Explosion Protection Systems David C. Kirby, Chair Dow Corporation, WV [U] Richard F. Schwab, Vice Chair Honeywell, Inc., NJ [U] Luke S. Morrison, Secretary Professional Loss Control Inc., Inc., Canada [SE]

Fountaintown, IN [SE] Joe R. Barton, Fountaintown, William J. Bradford, Brookfield, CT [SE] Reinhard E. Bruderer, Pred-Engineering Pred- Engineering Inc., FL [U] Rep. Ciba-Geigy Corporation Kenneth L. Cashdollar, U.S. National Institute Institute of Occupational Safety Safety & Health, Health, PA [RT] Kris Chatrathi, Fike Fike Corporation, MO [M] Henry L. Febo, Jr., FM Global, MA [I] Mark A. Fry, Mark A. A. Fry Fry & Associates, Inc., NJ [SE] Joseph P. Gillis, Westboro, MA [SE]

Safety & Design, Design, Inc., Inc., NJ [SE] Stanley S. Grossel, Process Safety Explosion Protection Protect ion L.P., FL FL [M] Dan A. Guaricci, ATEX Explosion Fire Suppression Systems, Systems, Inc., Inc., OH [IM] Michael D. Hard, Hard Fire Rep. Fire Suppression Systems Association David D. Herrmann, E.I. duPont de Nemours & Co., DE [U]

Omaha, NE [SE] Walter B. Howard, Omaha, Copyright NFPA

Canada [E] George Lobay, Canada Department of Natural Resources, Canada Amoco Corporation, IL [U] R. A. Mancini, BP Amoco Rep. American Petroleum Institute Steven A. McCoy, National Starch & Chemical Co., IN [U] Rep. NFPA Industrial Fire Protection Section

MA [I] Robert W. Nelson, Pocasset, MA Rep. Industrial Risk Insurers John Joseph Plunkett, U.S. Coast Guard, DC [E] Mitchel L. Rooker, BS&B Safety Systems, Systems, OK [M] Kevin Sheddrick, Engineered Engineered Storage Products Co., KS [M] Timothy F. Simmons, Eastman East man Kodak Company, Company, NY [U] Bill Stevenson, Cv Technology, Inc., FL [M] Stephen M. Stuart, Marsh USA, Inc., MI [I] Erdem A. Ural, Fenwal Safety Systems, MA [M] Roy A. Winkler, Solutia Solutia Inc., MO [U] Robert G. Zalosh, Worcester Polytechnic Institute, MA [SE]

Alternates

Union Carbide Carbide Corporation, WV [U] Laurence G. Britton, Union (Alt. to D. C. Kirby) Gary A. Chubb, Chubb Engineering, KS [M] (Alt. to K. Sheddrick) David G. Clark, E. I. duPont de Nemours & Co., DE [U] (Alt. to D. D. Herrmann) Ettore Contestabile, Canadian Explosives Research Resear ch Laboratory/CANMET, Laborat ory/CANMET, Canada Canada [E] (Alt. to G. Lobay) Thomas A. Gray, Akzo Nobel Inc., IL [U] (Alt. to S. A. McCoy)

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Edward J. Haas, Jr., Marsh USA, Inc., NY [I] (Alt. to S. M. Stuart) Paul F. Hart, Industrial Risk Insurers, IL [I] (Alt. to R. W. Nelson)

Inc., AZ AZ [IM] George A. Krabbe, Automatic Fire Controls, Inc., (Alt. to M. D. Hard) Arnold L. Mundt, BS&B Safety Systems, OK [M] (Alt. to M. L. Rooker) Samuel A. Rodgers, Honeywell, Inc., VA [U] (Alt. to R. F. Schwab) Joseph A. Senecal, Fenwal Safety Systems, MA [M] (Alt. to E. A. Ural)

Nonvoting

Jorda nstown, United Kingdom SE] Vladimir Molkov, University of Ulster at Jordanstown, Labor, WV WV Harry Verakis, U.S. Dept. of Labor, Sta ff Liaison Guy R. Colonna, NFPA Staff Committee Scope: This Committee shall have primary responsibility for documents on explosion protection systems for all types of equipment and for buildings, except pressure venting devices designed to protect against overpressure of vessels such as those containing flammable liquids, liquefied gases, and compressed gases under fire exposure conditions, as now covered in existing NFPA standards.

This list represents the membership at the time the Committee was balloted on the final text of this edition. Since that time, changes in the membership may have occurred. A key to classifications is found at the back of the document. NOTE: Membership on a committee shall not in and of itself constitute an endorsement of the Association or any document developed by the committee on which the member serves.

NFPA 69 Standard on Explosion Prevention Systems 2002 Edition

NOTICE: An asterisk (*) following following the number number or o r letter designating designating a paragraph indicates that explanatory material on the paragraph can be found in Annex A. A reference in brackets [ ] following a section or paragraph indicates material that has been extracted from another NFPA document. As an aid to the user, Annex F lists the complete Copyright NFPA

title and edition of the source documents for both mandatory and nonmandatory extracts. Editorial changes to extracted material consist of revising references to an appropriate division in this document or the inclusion of the document number with the division number when the reference is to the original document. Requests for interpretations or revisions of extracted text shall be sent to the appropriate technical committee. Information on referenced publications can be found in Chapter 2 and Annex F.

Chapter 1 Administration 1.1 Scope. (Reserved) 1.2 Purpose. 1.2.1 This standard shall cover the minimum requirements for installing systems for the prevention prevention of expl e xplosions osions in enclosures that contain flammab flammable le concentrations concentrat ions of flamm flammabl ablee gases, vapors, mists, dusts, or hybrid mixtures. 1.2.2 This standard shall provide basic information for design engineers, operating personnel, personnel, and authorities aut horities having having jurisdiction. jurisdiction. 1.3 Application. 1.3.1 This standard shall apply to systems and equipment used for the prevention of explosions by the prevention or control of deflagrations. 1.3.2 This standard shall not apply to following:

(1)

Devices Devices or systems systems designed designed to protect against against detonations

(2)*

Design, construct const ruction, ion, and installation of deflagration vents

(3)

Protection Protect ion against against overpressure due to phenomena phenomena other than internal internal deflagrations deflagrations

(4)

Chemical Chemical reactions other than combustion combustion processes

(5)

Unconfined deflagrations, deflagrat ions, such as open-air open-a ir or vapor cloud explosions

(6)

Rock dusting of coal mines, mines, as covered covere d by 30 CFR 75

(7)

General use of inert gas for fire extinguishment

(8)*

Preparation of tanks, piping, piping, or other enclosures for hot hot work, such such as cutting and welding

(9)

Ovens or furnaces handling handling flammable flammable or combustible atmospheres, atmospher es, as covered covere d by the following: (a)

NFPA 86, Standard 86, Standard for Ovens and Furnaces

(b)

NFPA 86C, Standard 86C, Standard for Industrial Furnaces Using a Special Processing Atmosphere

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(c)

NFPA 86D, Standard 86D, Standard for Industrial Furnaces Using Vacuum as an Atmosphere

(10)

Marine vapor control contr ol systems regulated regulat ed by 33 CFR 154

(11)

Marine vessel tanks regulated regulat ed by 46 CFR 30, 32, 35, and 39

1.4 Retroa Retroactivity. ctivity.

The provisions of this standard reflect a consensus of what is necessary to provide an acceptable degree of protection from the hazards addressed in this standard at the time the standard was issued. 1.4.1 Unless otherwise specified, the provisions of this standard shall not apply to facilities, equipment, structures, or installations that existed or were approved for construction or installation prior to the effective date of the standard. Where specified, the provisions of this standard shall be retroactive. 1.4.2 In those cases where the authority having jurisdiction determines that the existing situation presents an unacceptable degree of risk, the authority having jurisdiction shall be permitted permitted to apply retroactiv retroact ively ely any portions of this t his standard deemed appropriate. 1.4.3 The retroactive requirements of this standard shall be permitted to be modified if their application clearly would be impractical in the judgment of the authority having jurisdiction, and only where it is clearly evident that a reasonable degree of safety is provided. 1.5 Equivalency.

Nothing in the standard is intended to prevent the t he use of systems, methods, or devices devices of equivalent or superior quality, strength, fire resistance, effectiveness, durability, and safety over those prescribed by this standard. 1.5.1 Technical documentation shall be submitted to the authority having jurisdiction to demonstrate equivalency. 1.5.2 The system, method, or device shall be approved for the intended purpose by the authority having jurisdiction.

Chapter 2 Referenced Publications 2.1 General.

The documents or portions thereof listed in this chapter are referenced within this standard and shall be considered part of the requirements of this document. 2.2 NFPA Publications.

National Fire Prot ection Association, 1 Batterymarch Park, P.O. Box 9101, Quincy, Quincy, MA 02269-9101. NFPA 70, National 70, National Electrical Code® , 2001 , 2001 edition. Copyright NFPA

NFPA 72 ®, National Fire Alarm Code® , , 1999 edition. NFPA 86, Standard 86, Standard for Ovens and Furnaces, Furnaces, 1999 edition. NFPA 86C, Standard 86C, Standard for Industrial Furnaces Using a Special Processing Atmosphere, 1999 edition. NFPA 86D, Standard 86D, Standard for Industrial Furnaces Using Vacuum as an Atmosphere, 1999 Atmosphere, 1999 edition. NFPA 651, Standard 651, Standard for the Machining and Finishing of Aluminum and the Production and Handling of Aluminum Powders, Powders, 1998 1998 edition. 2.3 Other Publications. 2.3.1 ASME Publications.

American Society of Mechanical Engineers, Three Park Avenue, New York, NY 10016-5990. ASME Boiler ASME Boiler and Pressure Vessel Code, Code, Section VIII, 1998. ASME B31.3, Process B31.3, Process Piping , 1999. 2.3.2 ASTM Publication.

American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959. ASTM E 2079, Standard 2079, Standard Test Method for Limiting Oxygen (Oxidant) Concentration for Gases and Vapors, 2000. Vapors, 2000. 2.3.3 U.S. Government Publications.

U.S. Government Printing Office, Washington, DC 20402. Title 30, Code 30, Code of Federal Regulations, Regulations, Part 75. Title 33, Code 33, Code of Federal Regulations, Regulations, Part 154, “Waterfront Facilities.” Title 46, Code 46, Code of Federal Regulations, Regulations, Part 30. Title 46, Code 46, Code of Federal Regulations, Regulations, Part 32, “Shipping.” Title 46, Code 46, Code of Federal Regulations, Regulations, Part 35. Title 46, Code 46, Code of Federal Regulations, Regulations, Part 39.

Chapter 3 Definitions 3.1 General.

The definitions contained in this chapter shall apply to the terms used in this standard. Where terms are not included, common usage of the terms shall apply. Copyright NFPA

3.2 NFPA Official Definitions. 3.2.1* Approved. Acceptable to the authority having jurisdiction. 3.2.2* Authority Having Jurisdiction (AHJ). The organization, office, or individual responsible for approving equipment, materials, an installation, or a procedure. 3.2.3 Labeled. Equipment or materials to which has been attached a label, symbol, or other identifying mark of an organization that is acceptable to the authority having jurisdiction and concerned with product evaluation, that maintains periodic inspection of production of labeled equipment or materials, and by whose labeling the manufacturer indicates compliance with appropriate standards or performance in a specified manner. 3.2.4* Listed. Equipment, materials, or services included in a list published by an organization that is acceptable to the authority having jurisdiction and concerned with evaluation of products or services, that maintains periodic inspection of production of listed equipment or materials or periodic evaluation of services, and whose listing states that either the equipment, material, or service meets appropriate designated standards or has been tested and found suitable for a specified purpose. 3.2.5 Shall. Indicates a mandatory requirement. 3.2.6 Should. Indicates a recommendation or that which is advised but not required. 3.2.7 Standard. A document, the main text of which contains only mandatory provisions using the word “shall” to indicate requirements and which is in a form generally suitable for mandatory reference by another standard or code or for adoption into law. Nonmandatory provisions provisions shall be located in an appendix appendix or annex, annex, footnote, or fine-prin fine-printt note and are not to be considered a part of the requirements of a standard. 3.3 General Definitions. 3.3.1 Blanketing (or Padding). The technique of maintaining an atmosphere that is either inert or fuel-enriched in the vapor space of a container or vessel. 3.3.2 Burning Velocity. 3.3.2.1 Flame Burning Velocity. The burning velocity of a laminar flame under specified conditions of composition, temperature, and pressure for unburned gas. 3.3.2.2 Fundamental Burning Velocity. The burning velocity of a laminar flame under stated conditions of composition, temperature, and pressure of the unburned gas. [ 68:3.3] 3.3.3 Combustible. Capable of undergoing combustion. 3.3.4 Combustible Dust. Any finely divided solid material, 420 microns or smaller in diameter (material passing a U.S. No. 40 standard sieve), that presents a fire or deflagration hazard. [654:1.5] 3.3.5* Combustible Particulate Solid. A combustible solid material comprised of distinct particles or pieces, regardless of size, shape, or chemical chemical composition, composition, that is capable of being being pneumatically pneumatically conveyed. conveyed.

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3.3.6 Combustion. A chemical process of oxidation that occurs at a rate fast enough to produce heat and usually light light in the form of either a glow or flame. flame. 3.3.7 Concentration Reduction. 3.3.7.1 Combustible Concentration Reduction. The technique of maintaining the concentration of combustible material in a closed space below the lower flammable limit. 3.3.7.2 Oxidant Concentration Reduction. The technique of maintaining the concentration of an oxidant in a closed space below the concentration required for ignition to occur. 3.3.8 Deflagration. Propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium. [ 68:3.3] 3.3.9 Deflagration Pressure Containment. The technique of specifying the design pressure of a vessel and its appurtenances so they are capable of withstanding the maximum pressures resulting from an internal deflagration. 3.3.10 Deflagration Suppression. The technique of detecting and arresting combustion in a confined space while the combustion is still in its incipient stage, thus preventing the development of pressures that could result in an explosion. 3.3.11 Detonation. Propagation of a combustion zone at a velocity that is greater than the speed of sound in the unreacted medium. [ 68:3.3] 3.3.12 Explosion. The bursting or rupture of an enclosure or a container due to the development of internal pressure from a deflagration. 3.3.13 Fast-Acting Valve. A valve that closes a path of deflagration propagation in a pipe or duct in response to upstream detection of a deflagration. 3.3.14* Flame Arrester. A device that prevents the transmission of a flame through a flammable gas/air mixture by quenching the flame on the surfaces of an array of small passages through t hrough which the flame flame must pass. 3.3.15 Flame Front Diverter. A device that opens in response to the pressure wave preceding preceding the flame flame front of the t he deflagration, thereby t hereby venting venting the pressure wave and flame flame front. 3.3.16 Flame Speed. The speed of a flame front relative to a fixed reference point. Flame speed is dependent on turbulence, the equipment geometry, and the fundamental burning velocity. [68:3.3] 3.3.17* Flammable Limits. The minimum and maximum concentrations of a combustible material in a homogeneous mixture with a gaseous oxidizer that will propagate a flame. 3.3.17.1 Lower Flammable Limit (LFL). The lower flammable limit is the lowest concentration of a combustible substance in an oxidizing medium that will propagate a flame. 3.3.17.2 Upper Flammable Limit (UFL). The upper flammable limit is the highest concentration of a combustible substance in an oxidizing medium that will propagate a flame. 3.3.18 Flammable Range. The range of concentrations between the lower and upper

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flammable limits. [68:3.3] 3.3.19 Gas. The state of matter characterized by complete molecular mobility and unlimited expansion; used synonymously with the term vapor . [68:3.3] 3.3.19.1 Inert Gas. A gas that is noncombustible and nonreactive. 3.3.19.2 Purge Gas. An inert or a combustible gas that is continuously or intermittently added to a system to render the atmosphere nonignitible. 3.3.20 Hybrid Mixture. A mixture of a flammable gas with either a combustible dust or combustible mist. [68:3.3] 3.3.21 Inerting. A technique by which a combustible mixture is rendered nonignitible by adding an inert gas or a noncombustible dust. (See dust. (See also Blanketing.) 3.3.22* Isolation. A means of preventing certain stream properties from being conveyed past a predefined predefined point. 3.3.22.1 Chemical Isolation. A means of preventing flame front and ignition from being conveyed past a predetermined point by injection of a chemical suppressant. 3.3.22.2 Deflagration Isolation. A method employing equipment and procedures that interrupts the propagation of a deflagration flame front past a predetermined point. 3.3.22.3 Flow Isolation. A method employing equipment and procedures that interrupts flow and prevents pressure rise beyond a predetermined point. 3.3.22.4 Ignition Source Isolation. A method employing equipment and procedures that interrupts the propagation of an igniting medium past a predetermined point. 3.3.23* Limiting Oxidant Concentration (LOC). The concentration of oxidant below which a deflagration cannot occur. Materials other than oxygen can act as the oxidants. [86:2.2] 3.3.24 Liquid Seal. A device that prevents the passage of flame by passing the gas mixture through a noncombustible liquid. 3.3.25 Maximum Pressure (Pmax). The maximum pressure developed in a contained

deflagration for an optimum mixture. [ 68:3.3] 3.3.26 Mist. A dispersion of fine liquid droplets in a gaseous medium. [ 68:3.3] 3.3.27 Oxidant. Any gaseous material that can react with a fuel (either gas, dust, or mist) to produce combustion. combustion. Oxygen in air is the most common common oxidant. oxidant. [ 68:3.3] 3.3.28 Padding. See 3.3.1. 3.3.29 Pressure Piling. A condition during deflagration in which pressure increases in the unreacted medium ahead of the propagating combustion zone. 3.3.30 Spark Extinguishing System. An extinguishing system in which the radiant energy of a spark or an ember is detected and the spark or ember is quenched. 3.3.31 Suppress Suppressant. ant. The chemical agent used in a deflagration suppression system to

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extinguish the deflagration. 3.3.32 Vapor. See 3.3.19. 3.3.33 Ventilation. The changing of an atmosphere of any space by natural or mechanical means.

Chapter 4 General Requirements 4.1* Methods.

The methods recognized in this standard shall be grouped based on the prevention of combustion or on the prevention or limitation of damage after combustion occurs. 4.1.1 Methods Based on the Prevention of Combustion. The following shall be considered methods based on preventing combustion:

(1)

Oxidant Oxidant concentration reduction

(2)

Combustibl Combustiblee concentration reduction

4.1.2 Methods Based on the Prevention or Limitation of Damage. The following shall be considered methods based on preventing or limiting damage:

(1)

Spark extinguishing extinguishing systems

(2)

Deflagration Deflagration suppression

(3)

Isolation methods

(4)

Deflagration pressure pressur e containmen cont ainmentt

4.2 Limitations.

The limitations specific to each method shall be considered and are specified in the corresponding chapter for each method. 4.3 Factors to Be Considered.

The following factors shall be considered in the selection of one of the methods and the design of the system: (1)

Effectiveness of each method

(2)

Reliability Reliability of the system

(3)

Personnel Perso nnel hazards inherent in each method

4.3.1 The reliability of the system chosen shall be assessed using the following factors:

(1)

System design basis

(2)

Possibility Possibility of electrical electr ical and mechanical malfunction malfunction

(3)

Dependence on sophisticated sophisticat ed activating act ivating systems

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(4)

Need for special installation, traini tra ining, ng, operating, opera ting, testing, test ing, and maintenance procedures proc edures

(5)

Further limi limitations tations as presented in each chapter

4.3.2 In general, explosion prevention systems shall be used to protect processing, storage, and materials handling equipment. 4.3.3 When explosion prevention techniques are applied to rooms, buildings, or other enclosures where personnel are present, consideration shall be given to the safety of the personnel. personnel. 4.4 Plans. 4.4.1 Plans, system specifications, and manufacturer's recommendations for testing and maintenance shall contain information that enables the authority having jurisdiction to evaluate the explosion hazard and the effectiveness of the system. 4.4.2 Details of the plans shall include the following:

(1)

Pertinent Pert inent chemical and physical physical characteristics charact eristics of the materials involved involved

(2)

Location of hazards

(3)

Enclosures or limits limits and isolation of the hazards

(4)

Exposures to the hazards

4.5 Acceptance Test.

All new system installations and modifications shall be tested or otherwise evaluated to confirm the operational integrity of the system. 4.5.1 Tests shall be in accordance with the manufacturer's recommendations. 4.5.2 A written report of the tests shall be provided to the users. 4.6* Inspection and Maintenance.

inspected ed for operabil opera bility ity in in accordance with the manufacturer's manufacture r's 4.6.1* All systems shall be inspect recommendations. 4.6.2 An inspection and preventive maintenance schedule shall be established in accordance with the manufacturer's recommendations.

Chapter 5 Deflagration Prevention by Oxidant Concentration Reduction 5.1 Application.

The technique for oxidant concentration reduction for deflagration prevention shall be permitted permitted to be consi co nsidered dered where a mixture mixture of oxidant oxidant and flammabl flammablee material is confined confined to an enclosure within which the oxidant concentration can be controlled. Copyright NFPA

shall be be maintained maintained at an oxidant concentration concentr ation that is low enough to 5.1.1* The system shall prevent a deflagration. deflagration. 5.1.2 Oxidant concentration reduction shall be permitted to be applied to rooms or buildi buildings, ngs, but one of o f the t he following following shall apply, apply, since since oxygen-defi oxygen-deficien cientt atmospheres cannot sustain life:

(1)

Operations Operat ions in such areas shall be remotely remot ely controlled. contr olled.

(2)

Operating Operat ing personnel perso nnel shall be provided with breathing apparatus, appara tus, as well as other ot her safeguards.

5.2 Design and Operating Requirements. 5.2.1* Design Considerations. The following factors shall be considered in the design of a system intended to reduce the oxidant concentration:

(1)

Required Required reduction in oxidant oxidant concentration

(2)

Variations in the process, process temperature and pressure, and materials materials being being processed

(3)

Source Sourc e purge gas supply and equipment installation

(4)

Compatibility Compatibility of the purge gas with the process proc ess

(5)

Operating controls

(6)

Maintenance, inspection, and testing test ing

(7)

Leakage of purge gas to surrounding surrounding areas

(8)

Need for breathing breathing apparatus by personnel

5.2.2 Limiting Oxidant Concentrations (LOC).

and Table C.1(c) shall shall be permitted permitted to be used used as a 5.2.2.1* Table C.1(a), Table C.1(b), and basis basis for determining determining limitin limiting g oxidant oxidant concentrations of o f flammab flammable le gases or suspensions suspensions of of combustible dusts. 5.2.2.2 For fuel/inert/ fuel/inert/oxidant oxidant combinations combinations not not listed in Table C.1(a), C.1(a ), Table C.1(b), and Table C.1(c) or for situations when the process conditions differ from the conditions under which the existing data were obtained, the test methods described in ASTM E 2079, Standard Test Method for Limiting Oxygen (Oxidant) Concentration for Gases and Vapors , shall be permitted to be used. 5.2.2.3 The extent of oxidant reduction shall be determined by testing where conditions vary significantly from the test conditions under which the data were obtained. 5.2.3 Use of Purge Gas Systems. 5.2.3.1 An additional backflash prevention or protection system shall be installed if a purge gas system is used for lines collecting flammable mixtures and the collection system terminates at a flare or incinerator.

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5.2.3.2 Hard-piped vapor control systems shall not require flame arresters at each source connection to the system, provided that the system is designed to operate outside the flammable range. 5.2.3.3 Systems requiring hookups prior to vapor transfer, such as vapor collection from mobile vehicles, shall be purged to a level below the LOC prior to transfer, or backflash protection protect ion shall be provi pro vided ded near the point of o f connection. 5.2.3.4* Where oxygen-deficient oxygen-deficient atmospheres are maintained in equipment operating operat ing under conditions that might form pyrophoric iron sulfides or other pyrophoric materials, a procedure shall shall be developed to prevent uncontrolled oxi o xidation dation of o f the t he sulfides sulfides or other pyrophoric pyrophoric materials. materials. 5.3 Purge Gas Sources. 5.3.1 The purge gas shall be obtained from a source that is capable of continuously supplying the required amount of purge gas to maintain the necessary degree of oxidant deficiency. 5.3.2 Possible sources of purge gas shall include, but shall not be limited to, the following:

(1)

Commercially available available inert gas, such as nitrogen, nitroge n, carbon dioxide, argon, argo n, or helium, helium, supplied from high-pressure tanks or cylinders or from air separation plants

(2)

Inert gas supplied from a gas generator generat or that burns or catalyticall cata lytically y oxidizes a hydrocarbon to produce an oxygen-deficient purge gas

(3)

Products of combustion combustion from process furnaces or boiler boiler furnaces for which which purification purification or cooling cooling could be necessary to avoid contamination contamination

(4)*

Steam, if if it can be supplied supplied at a rate that raises and maintain maintainss the protected protect ed vessel vessel or system at a temperature high enough to prevent condensation of the steam

(5)

High-purity nitrogen nitroge n supplied by air oxidation of ammonia

(6)

Inert gas supplied by removal of oxygen from air by absorption, absorpt ion, adsorption, adsor ption, chemical reaction, or membrane permeation

(7)

Fuel gases such as methane methane or natural gas

5.4 Purge Gas Conditioning. 5.4.1 Purge gas shall be conditioned to minimize contaminants that might be harmful to the gas distribution system or that might interfere with the operation of the system. 5.4.2 Before introduction, the purge gas shall be at a temperature compatible with the process being being protected protect ed to t o mini minimi mize ze the chance of thermal ignition ignition or condensation. condensation. 5.4.3 Purge gas that is distributed in a system subject to freezing temperatures shall have a dew point such that water condensation cannot occur at the minimum ambient temperature to which the system will be exposed.

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5.5 Piping Systems.

Purge gas distribution systems shall be designed and installed in accordance with recognized engineering practices. 5.5.1 Where purge gas exceeds a gauge pressure of 15 psi (103 kPa), the piping system shall be designed in accordance accor dance with ANSI/ASME B31.3, Process B31.3, Process Piping. 5.5.2 Where required, piping systems shall be provided with filters, screens, or other means of preventing foreign material from entering critical parts of the system, such as pressure regulators, valves, and instrumentation. 5.5.3 Where required, moisture traps shall be provided and lines shall drain toward the traps. 5.5.3.1 Blowdown connections for moisture traps shall be provided. 5.5.3.2 Moisture traps shall be protected from freezing. 5.5.4 When flue gas or combustion gas is used, means shall be provided to prevent propagation of o f flame flame into the t he system being prot ected. 5.5.5* Manual shutoff valves valves shall be be provided at each ea ch major major division point in the distribution system. 5.5.6 The inert gas distribution system shall be designed to prevent contamination by hazardous process materials. 5.5.6.1 Where required, check valves or other design features shall be incorporated to prevent the t he potential pot ential for contamination contamination due to loss of purge gas supply or to excessive excessive pressure in the process unit unit being being protected. protect ed. 5.5.6.2 A single check valve shall not be considered a positive backflow connection. 5.5.7* Cross-connect Cross- connections ions between betwee n the purge gas distribution system and any any other system shall be prohibited unless one of the following criteria is met:

(1)

Positive measures shall be taken tak en to prevent backflow from the other ot her system into the purge gas system. system.

(2)

Cross-connections to backup purge gas systems systems shall shall be permitted permitted without without backflow backflow prevention unless unless backflow backflow could create creat e a hazard.

5.5.8 The entire distribution system shall be cleaned and functionally tested prior to being placed in service. 5.5.9 The gases from an enclosure or vessel being purged shall be vented to a safe location. 5.6 Application of Purge Gas at Points of Use.

Purge gas shall be introduced and exhausted so that distribution is ensured and the desired reduction in oxidant concentration is maintained throughout the system being protected. 5.6.1 Multiple inlets and outlets shall be permitted.

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5.6.2 Connections between the purge gas distribution piping and the protected enclosure or system shall be designed for maximum purge gas pressure. 5.7 Instrumentation. 5.7.1* General. Instrumentation shall be provided to monitor the purge gas being supplied to the distribution system. 5.7.1.1 Instrumentation shall be calibrated at scheduled intervals. 5.7.1.2 When the conditions being measured are critical to the safety of personnel, alarms shall be provided to indicate abnormal operation of the system. 5.7.2 Systems Operated Below the Limiting Oxidant Concentration (LOC).

Instr umentation n shall shall be be installed in as many points as necessary necessar y to ensure ensur e the 5.7.2.1* Instrumentatio desired oxidant concentration reduction within the protected system. 5.7.2.2 The determination of the LOC for the system shall be based on the worst credible case gas mixture yielding the smallest LOC. 5.7.2.3 A safety margin shall be maintained between the LOC and the normal working concentration in the system. 5.7.2.4* The safety margin shall take into account acc ount all of the following: following:

(1)

Fluctuations Fluctuations occurring in the system

(2)

Sensitivity and reliability reliability of monitoring and control contr ol equipment

(3)

Probability and consequences consequence s of an explosion

5.7.2.5 One of the following requirements shall be met where the oxygen concentration is continually monitored:

(1)

A safety margin of at least 2 volume percent perc ent below the worst wors t credible case LOC shall be maintained. maintained.

(2)

The LOC shall be less than 5 percent, percent , in which case, the equipment shall be operated opera ted at no more than 60 percent of the LOC.

5.7.2.6 The requirement requirement of 5.7.2.5 shall not apply to partial oxidation oxidation processes. 5.7.2.7 Where the oxygen concentration is not continuously monitored, all of the following requirements shall be met:

(1)

The oxygen oxygen concentration shall shall be designed designed to operate at no no more than 60 percent of the LOC, or 40 percent of the LOC if the LOC is below 5 percent.

(2)

The oxygen concentrat concent ration ion shall be checked on a regularly scheduled basis.

low-pre ssure field storage sto rage tanks ta nks that have padding shall shall not not 5.7.2.7.1* The vapor space in low-pressure require checking of the oxygen concentration. 5.7.2.7.2 The procedure of pulling a partial vacuum and then breaking the vacuum with inert

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gas shall be permitted without measuring the oxygen concentration if all of the following apply: (1)

The vacuum condition is held for a time to check for leakage.

(2)

The vacuum level is monitored.

(3)

The vacuum-creating vacuum-crea ting medium is compatible with the process proc ess chemistry.

(4)

The residual oxygen oxygen partial pressure is calculated calculated or demonstrated demonstrated by by test to be at least 40 percent below the LOC.

5.7.3 Systems Operated Above the Upper Flammable Limit (UFL). 5.7.3.1* Systems operating operat ing above above the UFL shall be permitted to be used, and the UFL shall be determi det ermined ned at the conditions conditions applicabl applicablee to the system. system. 5.7.3.2 Vent headers operated near atmospheric pressure shall be permitted to be rendered nonflammable by the addition of at least 25 volume percent of natural gas or methane where both of o f the t he following following criteria are met:

(1)

The vent headers shall shall not contain any vapor with a UFL greater than that of hydrogen in air (75 percent).

(2)

The vent headers shall not contain conta in oxygen in concentrat concent rations ions greater great er than can be derived from ambient air.

5.7.3.3 Instrumentation to control methane flow shall be acceptable to the authority having jurisdiction jurisdiction..

Chapter 6 Deflagration Prevention by Combustible Concentration Reduction 6.1* Application.

The technique for combustible concentration reduction shall be permitted to be considered where a mixture of a combustible material and an oxidant is confined to an enclosure and where the concentration of the combustible can be maintained below the lower flammable limit (LFL). 6.2 Basic Design Considerations. 6.2.1 All of the following factors shall be considered in the design of a system intended to reduce the combustible concentration below the lower flammable limit (LFL):

(1)

Required reduct ion in combustible concentrat concent ration ion

(2)

Variations in the process, process temperature and pressure, and materials materials being being processed

(3)

Operating controls

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(4)

Maintenance, inspection, and testing test ing

6.2.2* The lower flammable flammable limi limits ts of the combustibl co mbustiblee components shall be determined at all operating conditions, including startup and shutdown. 6.3 Design and Operating Requirements. 6.3.1 Combustible Concentration Limit. The combustible concentration shall be maintained at or below 25 percent of the lower flammable limit (LFL), unless the following conditions apply:

(1)

Where automatic auto matic instrumentation instrumentat ion with safety interlocks is provided, the combustible concentration shall be permitted to be maintained at or below 60 percent of the lower flammable limit.

(2)

Aluminum Aluminum powder production produc tion systems designed and operated opera ted in accordance accor dance with NFPA 651, Standard 651, Standard for the Machining and Finishing of Aluminum and the Production and Handling of Aluminum Powders, shall be permitted to be maintained at or below 50 percent of the lower flammable limit.

6.3.2* Catalytic Oxidation. Where catalytic oxidation is used for combustible concentration reduction, flame arresters shall be provided and the following requirements shall apply:

(1)

Flame arrester arre sterss shall be provided pro vided in all inlets inlets to the catalytic cat alytic oxidation unit.

(2)

Flame arrester arre sterss shall be periodically inspected and maintained. maintained.

6.3.3 Ventilation or Air Dilution. 6.3.3.1 If ventilation is used, the outlets from the protected enclosures shall be located so that hazardous concentrations of the exhausted air cannot enter or be drawn into the fresh air intakes of environmental air–handling systems. 6.3.3.2 Air intakes shall meet one of the following requirements:

(1)

They shall be located locat ed so that combustible material cannot enter the air-handling system, even in the event of spills or leaks.

(2)

They shall shall be provided with gas detectors detector s that automatically automatically interlock interlock to stop air intake.

6.3.3.3 Filters, dryers, or precipitators in the air intakes shall be located such that they are accessible for cleaning and maintenance. 6.4 Instrumentation. 6.4.1 Instrumentation shall be provided to monitor the control of the concentration of combustible components. 6.4.2 Instrumentation shall be calibrated at scheduled intervals. 6.4.3 Where the enclosure being protected presents a personnel hazard, alarms shall be provided to indicate indicate abnormal operation operat ion of the system. system.

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Chapter 7 Deflagration Prevention by Hot Particle Detection and Intervention Systems 7.1 Application.

Spark extinguishing systems shall be permitted to be considered for reducing the frequency of deflagrations in transport and receiving systems that handle combustible particulate solids. 7.1.1 Spark detection and extinguishing shall be used in conjunction with other explosion prevention prevention or explosion explosion protection prot ection measures, such as deflagration deflagration suppression or or deflagration venting, for those systems posing a dust explosion hazard. 7.1.2 Spark extinguishing systems shall be used for the detection and extinguishment of sparks or embers as they pass through ducts that transport combustible dusts or solids. 7.1.3 The spark extinguishing system shall operate by means of detectors that sense the radiation from a hot or glowing particle and actuate a special extinguishing system that quenches the particle. 7.1.4 Because the detection is by means of radiation, spark detection systems shall not be used in duct systems that have openings through which incident light could affect the detectors, unless the detectors are designed to be insensitive to visible light. 7.2 Limitations. 7.2.1 Spark extinguishing systems shall not be used for ducts designed to transport flammable gases. 7.2.2 Spark extinguishing systems shall not be used where the extinguishing agent creates a hazard. 7.2.3* Spark detectio det ection n and spark extinguishing systems shall shall be be limited limited to the detect ion and extinguishment of sparks or embers traveling at the system transport velocity. 7.2.4 Spark detection and spark extinguishing systems shall not be used in extinguishing deflagration flame fronts or flow isolation. 7.3 Spark Detection and Spark Extinguishing System Design Considerations. 7.3.1* General. Spark detection and spark extinguishing systems shall be listed or approved. 7.3.2 Detectors. 7.3.2.1 Spacing between a detector and the extinguishing agent injection point shall be based on all of the t he following: following:

(1)

Linear velocity of the material in the duct

(2)

Response time of the detector detecto r

(3)

Actuator circuitry. circuitry.

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7.3.2.2 The number of detectors shall be sufficient to detect a glowing particle at any location in the cross-sectional area of the duct. 7.3.2.3 Provisions shall be made to prevent obscuration of radiant energy detectors. 7.3.2.4 Detectors shall be protected from the accumulation of foreign material that would prevent functioning. functioning. 7.3.3 Power/Control Units. 7.3.3.1 A power/control unit with a minimum 24-hour standby battery backup shall be provided with each suppression system and shall supply energy to accomplish accomplish all of the following:

(1)

Power all detection devices devices

(2)

Energize all electrically electr ically actuated actu ated extinguishing systems

(3)

Energize visual and audible alarms

(4)

Transfer all auxiliary auxiliary control contr ol and alarm contacts conta cts

(5)

Control Contr ol system-disabling interlock and process proc ess shutdown shutdo wn circuits

7.3.3.2 The power/control unit shall meet the applicable requirements of 1.5.2 and Chapter 3 of NFPA 72, 72, National Fire Alarm Code®. 7.3.3.3 The power/control unit shall, as a minimum, fully and continuously supervise all of the following:

(1)

Wiring Wiring circuits for opens and other faults

(2)

AC power supply (primary)

(3)

Battery voltage, presence, and polarity

(4)

System safety interlock circuitry circuitr y

(5)

System-disabling System-disabling interlock circuitry

(6)

Releasing Releasing outputs

(7)

Electrical extinguish extinguishin ing g actuators actuato rs

(8)

Detectors

(9)

Visible Visible and audible alarms

(10)

Circuit ground fault

7.3.3.4 In addition to noncritical trouble alarms, the power/control unit shall have separate contacts capable of initiating an orderly shutdown of the protected process upon receipt of any trouble signal that indicates a disabled protection system. 7.3.3.5 The supervisory signal circuits shall be provided with a visual and audible trouble signal.

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7.3.4 Extinguishing System. 7.3.4.1 Discharge nozzles shall be located and arranged so that solid particles cannot obstruct the nozzles. 7.3.4.2 If water is used as the extinguishing agent, the water supply system shall be equipped with an in-line strainer. 7.3.4.3 The extinguishing agent supply system shall be capable of supplying all discharge nozzles at the rated volume and pressure. 7.3.4.4 The system shall contain enough extinguishing agent to provide for no less than 100 operations of the system. 7.3.4.5 An alarm shall sound when the pressure of the extinguishing agent falls below the minimum supply pressure specified by the manufacturer. 7.3.4.6 Auxiliary heating systems for extinguishing agent storage shall be provided, when necessary, and shall comply with all of the following:

(1)

The temperatur tempera turee of the extinguishing extinguishing agent shall be supervised.

(2)

An alarm shall sound at both the low and high temperatur tempera turee limits. limits.

7.3.5 Other Intervention Systems. Other intervention systems, including the following, shall be permitted to be actuated by the optical detection system:

(1)

Water deluge

(2)

Carbon dioxide flooding

(3)

Automatic fast-acting valves valves

(4)

Diverting valves

(5)

Steam Stea m snuffing snuffing

7.4 Testing.

A functional test of the extinguishing portion of the system shall be conducted in accordance with the manufacturer's specifications. 7.5 Spark Detection and Spark Extinguishing System Inspection and Maintenance.

Spark extinguishing systems shall be inspected and maintained in accordance with the manufacturer's recommendations. 7.5.1 A written report on the most recent inspection shall be kept on file for review. 7.5.2 The report shall include test and calibration data on all system components.

Chapter 8 Deflagration Control by Suppression

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8.1 Application. 8.1.1 The technique for deflagration suppression shall be permitted to be considered for most flammable gases, combustible mists, or combustible dusts that are subject to deflagration in a gas phase oxidant. 8.1.2 Enclosures that can be protected by a deflagration suppression system shall include, but shall shall not be limited limited t o, the following: following:

(1)

Processing Proc essing equipment, such as reactor react or vessels, mixers, mixers, blenders, pulverizers, mills, mills, dryers, ovens, filters, screens, and dust collectors

(2)

Storage equipment, equipment, such as atmospheric or low-pressure tanks, pressure tanks, and mobile mobile facilities facilities

(3)

Material-handli Mater ial-handling ng equipment, such as pneumatic and screw conveyors and bucket elevators

(4)

Laboratory Laborat ory and pilot plant equipment, including including hoods, glove boxes, test cells, cells, and other equipment

(5)

Aerosol Aeroso l fillin filling g rooms roo ms

8.2 Limitations. 8.2.1 Deflagration suppression is successful only where the suppressant can be distributed during the early stages of flame development. 8.2.2 Deflagration suppression is limited by the physical and chemical properties of the reactants in the system, as well as the design and construction of the enclosure. 8.2.3 The strength of the protected enclosure shall be greater than the maximum suppressed deflagration pressure (including effects of suppressant discharge). 8.3 Personnel Safety. 8.3.1* Disarming and Lockout/Tagout Procedures. 8.3.1.1 Disarming and lockout/tagout procedures shall be followed prior to entering equipment protected by deflagration suppression systems. 8.3.1.2 The deflagration suppression system shall be disarmed prior to performing maintenance operations on the protected equipment if discharging the suppressant could result in injury. 8.3.1.3 Operation of the protected equipment shall be interlocked through the suppression system control panel so that operation cannot be resumed until the suppression system is armed. 8.3.2 Training. Personnel shall be trained in the safety procedures that are to be carried out prior to, during, during, and after maintenance. maintenance. 8.4 Basic Design Considerations.

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8.4.1 General. The design of a deflagration suppression system shall include, but shall not be limited to, the following:

(1)

Deflagration Deflagration characteristics of the combustibl combustiblee material

(2)

Identification of equipment to be protect prot ected ed and its design specifications

(3)

Detection technique technique

(4)

Suppressant

(5)

Installation, Installation, operation, and test procedures

8.4.2 Process Analysis. 8.4.2.1 A thorough analysis of the process shall be conducted to determine the type and degree of deflagration hazards inherent in the process. 8.4.2.2 Factors such as the type of combustible, the internal geometry and total volume to be protected, pro tected, and the t he operating o perating conditions shall be reviewed in detail. 8.4.2.3 The potential malfunctions that could affect the extent of the deflagration hazard also shall be determined. 8.4.3 Actuation of Other Devices and Systems. The deflagration suppression system shall be permitted to actuate other devices devices and systems such as high-speed high-speed isolation valves, valves, rapid pneumatic pneumatic conveyin conveying g system shutdowns, or deflagration deflagration vents. 8.4.4 Drawings and Design Calculations. 8.4.4.1 Drawings and design calculations shall be developed for each system. 8.4.4.2 Calculations of the final reduced deflagration pressures shall be provided. 8.5 Power/Control Units. 8.5.1 A power/control unit with a standby battery backup of no less than 24 hours shall be provided with each suppression system and shall supply energy to accomplish accomplish the following following::

(1)


(2)

Energize all electrically electr ically fired initiators

(3)


(4)


(5)


8.5.2 The power/control unit shall meet the applicable requirements of 1.5.2 and Chapter 3 of NFPA 72, 72, National Fire Alarm Code. Code. 8.5.3 The power/control unit shall, as a minimum, fully and continuously supervise the following:

(1)


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(2)


(3)


(4)


(5)


(6)


(7)

Electrically Electr ically fired initiat initiators ors

(8)

Detectors

(9)

Visual and audible alarms

(10)


8.5.4 In addition to noncritical trouble alarms, the power/control unit shall have separate contacts capable of initiating an orderly shutdown of the protected process upon receipt of any trouble signal that indicates a disabled protection system. 8.5.5 The supervisory signal circuits shall be provided with a visual and audible trouble signal. 8.6 Detectors. 8.6.1 The deflagration shall be detected by sensing either the pressure increase or the radiant energy from the combustion process. 8.6.2 Provisions shall be made to prevent obscuration of radiant energy detectors. 8.6.3 Detectors shall be protected from the accumulation of foreign material that would prevent functioning. functioning. 8.7 Electrically Fired Initiators. 8.7.1 Electrically fired initiators shall be mounted so that their maximum temperature rating, as specified by the manufacturer, is not exceeded. 8.7.2 A source of electrical energy shall be used so that the firing characteristics of the initiators do not deviate from the manufacturer's specifications. 8.8* Suppressant and Suppressant Storage Containers. 8.8.1 The suppressant shall be compatible with the combustible material in the protected enclosure. 8.8.2 The suppressant shall be effective at the expected extremes of temperature encountered in the protected enclosure. 8.8.3 Means shall be provided to verify the pressure of the pressurized reservoirs. 8.9 Installation.

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8.9.1 All components of the system shall be mounted in the location and in the manner specified by the system designer. 8.9.2 Suppressant discharge nozzles shall be mounted in such a way as to prevent damage or failure to any appurtenances or fixtures in the enclosure protected. 8.9.3 Means shall be used to protect detectors and suppressant discharge devices from accumulating foreign material that would prevent functioning. 8.9.4 Terminals and mechanical parts shall be protected from moisture and other contaminants. 8.9.5 The temperatures at mounting locations shall not exceed the maximum operating temperatures of system components. 8.10 Electrical. 8.10.1 Wiring for the control circuits shall be isolated and shielded from all other wiring to prevent possible induced currents. 8.10.2 Where environmental conditions warrant, conduits shall be sealed to prevent the entrance of moisture and other contaminants. 8.10.3 Where a conduit is used for wiring multiple installations, the wiring for each suppression system shall be run in separate conduits or wired with shielded cables run in common conduits. 8.10.4 All wiring shall shall meet meet the t he applicable applicable requirements of o f NFPA 70, National 70, National Electrical Code. Code. 8.11 Inspection and Maintenance of Deflagration Suppression Systems.

detect ors. 8.11.1* Equipment shall be designed to allow inspection of nozzles and detectors. 8.11.2 Suppression systems shall be inspected and tested at 3-month intervals by personnel trained by the system's manufacturer. 8.11.3* Containers Containers of suppressant shall shall be checked checked for pressure and loss loss of agent. 8.11.4 A container having a pressure (corrected for temperature) that is less than the minimum value specified by the manufacturer shall be reconditioned or replaced. 8.11.5 Detectors shall be tested and calibrated as necessary to meet system specifications. 8.11.6 System interlocks shall be verified for functioning. 8.11.7 The control unit shall be tested to ensure that the system functions as required and that all external circuits are supervised. 8.11.8 A written report on the most recent inspection shall be kept on file for review. 8.11.9 The report of the most recent inspection shall include test and calibration data on all system components.

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8.11.10 Extinguishing agent cylinders shall be hydrostatically tested consistent with DOT requirements. 8.12 Procedures After System Actuation.

In the event of system system actuation, actu ation, inspection and testing test ing as specified in in 8.11.2 shall be performed before t he system is returned to service.

Chapter 9 Deflagration Control by Isolation 9.1 Application. 9.1.1* The technique for deflagrat deflagration ion isolation isolation shall be permitted to be considered for interruption or mitigation of flame, deflagration pressures, pressure piling, and flame-jet ignition between equipment that is interconnected by pipes or ducts. 9.1.2 One or more of the technologies described in this chapter shall be permitted to be used with other explosion prevention systems described in this standard, in addition to, or in conjunction with, deflagration venting. 9.1.3 Isolation methods shall be used to prevent the passage of, to arrest, to divert, or to extinguish the deflagration flame front and, in some cases, the combustion-generated pressure. 9.1.4 Isolation shall be permitted to be used for flammable gases or combustible dust systems.

Isolat ion system system design shall be permitt permitted ed to be based on various techniques that 9.1.5* Isolation include, but are not limited to, the use of the following: (1)

Rotary Rota ry valves

(2)

Flame Flame arresters

(3)

Automatic fast-acting valves valves

(4)

Flame Flame front diverters diverters

(5)

Flame front extinguishing extinguishing systems

(6)

Liquid seals

(7)

Spark detection dete ction and spark extinguishing extinguishing systems

9.1.6 The strength of piping, ducts, and enclosures in an isolation system shall be designed to withstand anticipated pressures. 9.2* Rotary Valves. 9.2.1 Rotary valves shall be used only for systems handling combustible dust. 9.2.2 Rotary valves intended for deflagration isolation systems shall be designed as follows:

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(1)

A clearance that is is small enough to prevent the passage of flame shall be provided between the t he rotor r otor and the t he valve housing.

(2)

At least two vanes on each side of the valve housing shall be in a position of minimum clearance at all times.

9.2.3 Rotary valves shall be capable of withstanding the maximum expected pressure. 9.2.4* Rotary Rot ary valves valves intended for deflagration isolation systems shall shall have have metal bodies and and vanes unless it is shown by test data that nonmetallic or composite materials prevent flame passage. 9.3* Flame Arresters.

Sect ion 9.3 shall not apply to the following: 9.3.1 Section (1)

Devices that utilize a liquid seal to prevent the passage of flame flame

(2)

Devices that rely on gas flow velocity to prevent upstream upstr eam propagation propa gation of flame flame

(3)

Systems handling handling combustible dusts

9.3.2* Flame arrester arre sterss shall be placed in the potential pot ential flame flame path between the source sourc e of ignition and the system to be protected. 9.3.3 Flame arresters shall be installed in accordance with the manufacturer's instructions.

arre sterss for in-line in-line use shall be tested teste d for such an applicatio application. n. 9.3.4* Flame arrester 9.3.5* An in-line in-line arrester arrest er that experiences continued cont inued burning burning for for a time longer than that for which it was tested shall meet the following criteria:

(1)

A means of detecting dete cting the burning shall be provided on both sides of the arrester arre ster along with an alarm or automatic device to interrupt flow prior to failure.

(2)

If thermocouples thermoc ouples are used, they shall not be placed in thermowells. thermowe lls.

9.3.6* Arresters Arrest ers shall be inspecte inspected d periodically, periodically, based on facility facility experience, and after each incident where they have been called upon to function. 9.3.6.1 The inspection shall determine whether any damage has occurred that could affect the performance of the device. 9.3.6.2 Damaged components shall be replaced. 9.4* Automatic Fast-Acting Valve Systems. 9.4.1 Automatic fast-acting valve systems shall be designed to detect a deflagration and to prevent propagation pro pagation of flame flame and combustion-generated combustion-generated pressure pr essure beyond beyond the t he fast-acting fast-act ing valves by providing a positive mechanical seal. 9.4.2 Factors that affect the performance of fast-acting automatic-closing valves shall be considered in the design and applications and shall include, but not be limited to, the following:

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(1)


(2)

Volume, Volume, configuration, configuration, and operating characteristics of the vessel

(3)

Type of deflagration protection prot ection used on the vessel and piping

(4)

Volume, Volume, length, length, cross-sectional area, configuration, configuration, and strength of the piping piping

(5)

Velocity of the combustible air mixtur mixturee in the pipe

(6)

Location of system components components

(7)

Closure time of the valve, including including control cont rol and detection dete ction components

(8)

Detection technique technique

9.4.3 Fast-acting valves and deflagration detectors shall be capable of withstanding the maximum expected deflagration pressures, including pressure piling. 9.4.4* The fast-acting fast-ac ting valve valve systems shall shall be be of a design that has been tested test ed under deflagration conditions to verify their performance. 9.4.5 Spacing between a detector and the fast-acting valve shall be based on the maximum flame speed expected in the duct and the response time of the detector, the valve, and the actuator circuitry. 9.4.6 The diameter of the pipe leading to the automatic fast-acting valve shall not be decreased, unless the automatic fast-acting valve has been specifically tested for the configuration. 9.4.7 Personnel shall be trained in the safety procedures that are to be carried out prior to, during, and after maintenance. 9.4.8 A power/control unit with a minimum 24-hour standby battery backup shall be provided with each suppression system and shall supply energy to accomplish accomplish the following following::

(1)


(2)

Energize all electrically electr ically actuated actu ated valve systems

(3)


(4)


(5)

Control Contr ol system–disabling system–disabling interlock and process proc ess shutdown shutdo wn circuits

9.4.8.1 The power/control unit shall meet the applicable requirements of 1.5.2 and Chapter 3 of NFPA 72, 72, National Fire Alarm Code. Code. 9.4.8.2 The power/control unit shall, as a minimum, fully and continuously supervise the following:

(1)


(2)


(3)


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(4)


(5)


(6)


(7)

Electrically Electr ically actuated actu ated valves

(8)

Detectors

(9)


(10)


9.4.8.3 The supervisory signal circuits shall be provided with a visual and audible trouble signal. 9.4.8.4 In addition to noncritical trouble alarms, the power/control unit shall have separate contacts capable of initiating an orderly shutdown of the protected process upon receipt of any trouble signal that indicates a disabled protection system. 9.4.9 The deflagration shall be detected by sensing either the pressure increase or the radiant energy from the combustion process. 9.4.9.1* Provisions Provisions shall shall be made made to prevent obscuration of radiant energy detectors. detecto rs. 9.4.9.2 Detectors shall be protected from the accumulation of foreign material that would prevent functioning. functioning. 9.4.10 Electrically fired initiators shall be mounted so that their maximum temperature rating, as specified by the manufacturer, is not exceeded. 9.4.11 Pneumatic valve actuator systems shall comply with the following requirements:

(1)

Pneumatic valve actuato act uators rs shall be mounted so that their maximum maximum temperature tempera ture rating, as specified by the manufacturer, is not exceeded.

(2)

Means shall shall be provided to verify verify the pressure of the pressurized reservoir for the pneumatic pneumatic valve valve actuator. actuato r.

9.4.12 Wiring for the control circuits shall be isolated and shielded from all other wiring to prevent possible induced currents. 9.4.12.1 Where environmental conditions warrant, conduits shall be sealed to prevent the entrance of moisture and other contaminants. 9.4.12.2 Where a conduit is used for wiring multiple installations, the wiring for each automatic fast-acting valve system shall be run in a separate conduit. Alternatively, each system shall be permitted to be wired with shielded cables run in common conduit.

shall meet meet the t he applicable applicable requirements of o f NFPA 70, National 70, National Electrical 9.4.12.3 All wiring shall Code. Code. 9.4.13 All components of the system shall be mounted in the location and in the manner specified by the system designer.

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9.4.13.1 Where necessary, measures shall be used to protect detectors and fast-acting valves from accumulating foreign material that would prevent operation. 9.4.13.2 Terminals and mechanical parts shall be protected from moisture and other contaminants. 9.4.13.3 The temperatures at mounting locations shall not exceed the maximum operating temperatures of system components. 9.4.14 Inspection and maintenance of automatic fast-acting valve systems shall comply with the following requirements:

(1)

Automatic Automat ic fast-acting fast-a cting valve systems shall be inspected and maintained in accordance accor dance with the manufacturer's recommendations.

(2)

Containers Containers of suppressant shall shall be checked for pressure and loss of agent.

(3)

A container having having a pressure (corrected (correct ed for temperature) that is less than the minimum value specified by the manufacturer shall be reconditioned or replaced.

(4)

A written report on the most recent inspection inspection shall shall be kept on file file for review. The report shall include test and calibration data on all system components.

9.4.15 In the event of system actuation, inspection and testing as specified by the manufacturer shall be performed before the system is returned to service. 9.5* Flame Front Diverters. 9.5.1 Flame front diverters shall be permitted to be used as a deflagration loss control measure. 9.5.2 Flame front diverter system design considerations shall include, but not be limited to, the following:

(1)


(2)

Volume, Volume, configuration, configuration, and operating characteristics of the equipment equipment to be protected protect ed and the t he conveying conveying system

(3)

Type of deflagration deflagration protection protect ion used on the vessel

(4)

Length, cross-sectional area, configuration, configuration, and strength of the piping piping

(5)

Velocity of the combustible air mixtur mixturee in the pipe

(6)

Location of the flame flame front diverter diverter and its associated piping piping

(7)

Turbulence-generating Turbulence-ge nerating features featur es in the piping such as fittings, valves, elbows, and wall roughness

(8)

Location Locat ion of probable ignition sources sourc es

9.5.3 The body design shall divert the flame front to atmosphere and away from the downstream piping.

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9.5.4 The body shall be capable of withstanding expected deflagration pressure. 9.5.5 The closure device shall be either a rupture disc or a cover plate. 9.5.6 Where the closure device could be a missile hazard, it shall be either tethered or contained in a cage. 9.5.7 The hazard of flame discharge from the flame front diverter shall be considered when designing the placement of the device. 9.5.8 The flame front diverter shall discharge to a safe, unrestricted, outdoor location. 9.5.9* Flame Flame front diverters shall shall be tested for the application. application. 9.6 Chemical Isolation Systems. 9.6.1 General Requirements. 9.6.1.1 Chemical isolation systems shall be permitted to be used to isolate interconnected process volumes volumes from the effects of deflagration deflagration flame passage through interconnecting interconnecting pipe. 9.6.1.2 Chemical isolation systems shall be designed to detect a deflagration flame event and to cause discharge of an extinguishing agent into a length of pipe sufficient to prevent flame propagation past the point of o f agent discharge. discharge. 9.6.1.3 Chemical isolation system components exposed to the process environment shall be capable of withstanding the maximum expected deflagration pressure. 9.6.1.4 A chemical isolation system shall be of a design that has been tested under deflagration conditions to verify performance. 9.6.1.5 The distance between the position of a deflagration flame event detector and the associated agent discharge point shall be based on the following:

(1)

Maximum Maximum deflagration flame speed expected expect ed in the pipe

(2)

Response time characteristics of the detector detecto r

(3)

Discharge Discharge rate from the agent containers

9.6.1.6 Chemical isolation systems shall be disarmed before maintenance operations are performed on the system components. 9.6.1.7 Personnel shall be trained in safety procedures to be carried out prior to, during, and after maintenance. 9.6.2 Detectors. 9.6.2.1 Deflagration flame event detectors shall be of the pressure sensing or radiant energy sensing type. 9.6.2.2 Provisions shall be made to prevent obscuration of radiant energy detectors. 9.6.2.3 Provisions shall be made to prevent blockage of access to the sensing surface of pressure-type detectors. det ectors.

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9.6.3 Extinguishing Agent and Containers. 9.6.3.1 The extinguishing agent shall be chemically compatible with the material normally conveyed through the pipe system being protected. 9.6.3.2 The extinguishing agent shall be of a type that is effective at all temperatures to be encountered in the application. 9.6.3.3 Extinguishing agent containers, if used as shipping containers, shall be designed to meet the requirements of the U.S. Department of Transportation. 9.6.3.3.1 If not used as shipping containers, extinguishing agent containers shall be designed, fabricated, inspected, certified, and stamped in accordance with Section VIII of the ASME Boiler ASME Boiler and Pressure Vessel Code. Code. 9.6.3.3.2 The design pressure shall be suitable for the maximum pressure developed at 55°C (130°F) or at the maximum controlled temperature limit. 9.6.4 Electrically Fired Initiators. Electrically fired initiators shall be mounted so that their maximum temperature rating, as specified by the manufacturer, is not exceeded. 9.6.5 Power/Control Units. 9.6.5.1 A power/control unit with a minimum 24-hour standby battery backup shall be provided with each chemical chemical isolation isolation system and shall supply energy to accomplish accomplish the following:

(1)


(2)

Energize all electrically electr ically actuated actu ated chemical isolation systems

(3)


(4)


(5)


9.6.5.2 The power/control unit shall meet the applicable requirements of 1.5.2 and Chapter 3 of NFPA 72, 72, National Fire Alarm Code. Code. 9.6.5.3 The power/control unit shall, as a minimum, fully and continuously supervise the following:

(1)


(2)


(3)


(4)


(5)


(6)


(7)

Electrical extinguish extinguishin ing g actuators actuato rs

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(8)

Detectors

(9)


(10)


9.6.5.4 The supervisory signal circuits shall be provided with a visual and audible trouble signal. 9.6.5.5 In addition to noncritical trouble alarms, the power/control unit shall have separate contacts capable of initiating an orderly shutdown of the protected process upon receipt of any trouble signal that indicates a disabled protection system.

contro l unit unit shall meet applicable requirements require ments of NFPA 70, 9.6.5.6 The power and control National Electrical Ele ctrical Code. Code. 9.6.6 Electrical. 9.6.6.1 Wiring for the control circuits shall be isolated and shielded from all other wiring to prevent possible induced currents. 9.6.6.2 When a conduit is used for wiring multiple installations, the wiring for each chemical isolation system shall be run in separate conduit or wired with shielded cables run in common conduit.

shall meet meet the t he applicable requirements of o f NFPA 70, National 70, National Electrical 9.6.6.3 All wiring shall Code. Code. 9.6.7 Installation of Chemical Isolation Systems. 9.6.7.1 All components of the system shall be mounted in the location and in the manner specified by the system designer. 9.6.7.2 Where necessary, measures shall be used to protect detectors and extinguisher components from accumulating foreign material that would prevent operation. 9.6.7.3 Terminals and mechanical parts shall be protected from moisture and other contaminants. 9.6.7.4 The temperatures at mounting locations shall not exceed the maximum operating temperatures of system components. 9.6.8 Inspection and Maintenance of Chemical Isolation Systems. 9.6.8.1 Chemical isolation systems shall be inspected and maintained in accordance with the manufacturer's recommendations. 9.6.8.2 Containers of suppressant shall be checked for pressure and loss of agents. 9.6.8.3 A container having a pressure (corrected for temperature) that is less than the minimum value specified by the manufacturer shall be reconditioned or replaced. 9.6.8.4 A written report on the most recent inspection shall be kept on file for review. 9.6.8.5 The report shall include test and calibration data on all system components.

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9.6.9 Procedures After System Trouble or Actuation. In the event of system actuation, inspection and maintenance maintenance as specified spe cified in 9.6.8 shall be performed before the system is returned to service. 9.7* Liquid Seals.

A liquid seal shall be used for preventing the passage of flame by passing gas through a liquid. 9.7.1 Liquid seal devices shall be designed for the gases being handled at the flow velocities range in the system and to withstand the maximum anticipated deflagration pressure. 9.7.2 Liquid seals shall be designed in accordance with other recognized practices. 9.7.3* Means for providi pro viding ng and maintaining maintaining the liquid liquid level shall shall be provided, as well as an alarm to detect malfunction.

Chapter 10 Deflagration Control by Pressure Containment 10.1 Application. 10.1.1 The technique for deflagration pressure containment shall be permitted to be considered for specifying the design pressure of a vessel and its appurtenances so they are capable of withstanding the maximum pressures resulting from an internal deflagration. 10.1.2 This chapter shall provide the basis for determining the vessel design pressure required to withstand the pressures resulting from an internal deflagration. 10.1.3 This chapter shall be limited to systems in which the oxidant is air. 10.1.4 The design pressure specified by this chapter shall be based on the most severe set of system conditions that can occur.

pressur e containment shall shall be applied to a vessel with with attached atta ched 10.1.5* Deflagration pressure equipment to protect such equipment from imposed pressure loads that could equal or be greater than the pressure loads experienced by the protected vessel. 10.2 Design Limitations. 10.2.1* Deflagration pressure pressur e containment techniques shall not be applied to systems for the purpose of containing containing a detonation. 10.2.2* Deflagration Deflagration pressure containment containment shall not be applied applied to systems systems where two or more vessels are connected by large-diameter pipes or ducts, unless one of the following criteria is met:

(1)

Deflagration Deflagration pressure containment containment shall shall be permitted permitted to be used where interconnected piping is provided with deflagration isolation.

(2)

Deflagration pressure pressur e containmen cont ainmentt shall be permitted permitte d to be used where venting is provided for interconnected interconnected piping. piping.

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(3)

Deflagration Deflagration pressure containment containment shall shall be permitted permitted to be used where interconnected vessels are designed to contain the increased pressures due to the effects of pre-pressurization.

(4)

Deflagration isolation or venting of one vessel shall be permitted permitte d to be used.

(5)

Deflagration pressure pressur e containmen cont ainmentt shall be permitted permitte d to be used for initial initial gauge pressures exceeding exceeding 2 bar (30 psi) only when the maximum maximum deflagration deflagration pressure ratio (R) is (R) is determined by test or calculations.

10.2.3* The alternative alternative of 10.2.2(5) shall shall not not be permitted permitted where test data are availabl available. e. 10.3 Design Bases. 10.3.1 Vessels designed for deflagration pressure containment shall be designed and constructed according to Section VIII, Division 1, of the ASME Boiler and Pressure Vessel Code, Code, which takes into consideration sources of overpressure other than deflagration.

pressur e of the vessel, as calculated in 10.3.3, shall be based either on 10.3.2 The design pressure preventing preventing rupture rupt ure of the vessel (the ultimate ultimate strength of the t he vessel) or on preventi pr eventing ng permanent permanent deformation deformation of o f the t he vessel (t he yield yield strength of the t he vessel) from internal positive overpressure. Due to the vacuum that could follow a deflagration, all vessels whose deflagration pressure containment design is based on preventing deformation also shall be designed to withstand an absolute internal pressure of 68.95 kPa (10 psi), or they shall be provided with vacuum relief. pressur e shall be calculated according to the following equations: equat ions: 10.3.3* The design pressure

(10.1)

(10.2)

where: P f = design pressure to prevent rupture due to internal deflagration (psig) Pd = design pressure to prevent deformation due to internal deflagration (psig) Pi = maximum initial pressure at which combustible atmosphere exists (psig) R = R = ratio of maximum deflagration pressure, in absolute pressure units, to maximum initial pressure, inconsistent absolute pressure units Fu = ratio of the ultimate stress of the vessel to allowable stress of the vessel F y = ratio of the yield stress of the vessel to allowable stress of the vesselFor U.S. Customary units, 6.89 kPa =1 psi 10.3.3.1* The dimensionless dimensionless ratio, rat io, R, is the ratio of the maximum deflagration pressure, in absolute pressure units, to the maximum initial pressure, in consistent absolute pressure units.

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10.3.3.2 For use as a practical design basis (since optimum conditions seldom exist in industrial equipment), the value of R shall R shall be as follows:

(1)

For most gas/air mixtures, mixtures, the value value of R shall R shall be 9.

(2)

For St-1 and St-2 dust/air mixtures, mixtures, the value value of R shall R shall be 11.

(3)

For St-3 dust/air mixtures, mixtures, the value value of R shall R shall be 13.

10.3.3. 2 shall be permitted to be 10.3.3.3 A value for R other than the values specified in 10.3.3.2 used if such value can be substantiated by test data or calculations. 10.3.3.4 The vessel design pressure shall be based on the wall thickness of the vessel, excluding any allowance for corrosion or erosion. 10.3.3.5 For operating temperatures below 25°C (77°F), the value of R' of R' shall shall be calculated for use in Equation 10.1 and Equation 10.2:

(10.3)

where: R = R = maximum deflagration ratio for the mixture measured at 25°C (77°F) Ti = operating temperature (°C) 10.3.4 The presence of any pressure relief device on the system shall not cause the design pressure calculated calculated by the methods of 10.3.3 to be reduced. r educed.

maximum pressure pressur e for positive pressure pressur e systems shall shall be be as follows: follows: 10.3.5* The maximum (1)

For positive pressure pressur e systems handling handling gases and liquids, the maximum maximum initial initial pressure, P , shall , i shall be the maximum initial pressure at which a combustible atmosphere is able to exist, but a pressure not higher than the setting of the pressure relief dev ice plus its accumulation. accumulation.

(2)

(3)

For positive pressure pressur e systems handling handling dusts, dusts , the maximum maximum initial initial pressure pressur e shall be the greater of the following two pressure values: (a)

Maximum Maximum possible discharge pressure of the compressor compresso r or blower blower that is is suspending or transporting the material

(b)

Setting of the pressure relief relief device device on the vessel being being protected plus its its accumulation

For gravity discharge of dusts, the maximum maximum initial pressure pressur e shall be the atmospheric atmospher ic gauge pressure (0.0 bar or 0.0 psi).

10.3.6 For systems operating under vacuum, the maximum initial pressure shall not be less than atmospheric gauge pressure (0.0 bar or 0.0 psi). 10.3.7 The vessel design shall take into consideration the minimum operating temperature at which a deflagration could occur, which shall be compared with the temperature characteristics of the vessel's construction material to ensure that brittle fracture cannot

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result from a deflagration. 10.3.8 Auxiliary equipment such as vent systems, manways, fittings, and other openings into the vessel shall be designed to ensure integrity of the total system and shall be inspected periodicall periodically. y. 10.4 Maintenance.

Relief devices shall be inspected periodically to ensure that they are not plugged, frozen, or corroded. 10.5 Threaded Fasteners.

Threaded fasteners on vessel appurtenances shall be inspected to ensure that design pressure ratings are maintained. 10.6 Inspection After a Deflagration.

Any vessel designed to contain a deflagration that experiences a deflagration shall be inspected to verify that the vessel is still serviceable for its intended use.

Annex A Explanatory Material Annex A is not a part of the requirements of this NFPA document but is included for informational purposes only. This annex contains explanatory material, numbered to correspond with the applicable text paragraphs. information n on deflagrat deflagration ion venting, see NFPA 68, Guide 68, Guide for Venting of A.1.3.2(2) For informatio Deflagrations. Deflagrations. information n on cutting and welding practices, pract ices, see NFPA 51B, Standard 51B, Standard for A.1.3.2(8) For informatio Fire Prevention During Welding, Cutting, and Other Hot Work . For information on preparation of o f tanks, t anks, piping, piping, or other enclosures for hot work, see NFPA 326, Standard for the Safeguarding of Tanks and Containers for Entry, Cleaning, or Repair . Repair . A.3.2.1 Approved. The National Fire Protection Association does not approve, inspect, or certify any installations, procedures, equipment, or materials; nor does it approve or evaluate testing laboratories. In determining the acceptability of installations, procedures, equipment, or materials, the authority having jurisdiction may base acceptance on compliance with NFPA or other appropriate standards. In the absence of such standards, said authority may require evidence of proper installation, procedure, or use. The authority having jurisdiction may also refer to the listings or labeling practices of an organization that is concerned with product evaluations evaluations and is thus in a position to determine determine compliance compliance with appropriate appropr iate standards for the current production of listed items. A.3.2.2 Authority Having Jurisdiction (AHJ). The phrase “authority having jurisdiction,” or its acronym AHJ, is used in NFPA documents in a broad manner, since jurisdictions and approval agencies vary, as do their responsibilities. Where public safety is primary, the authority having jurisdiction may be a federal, state, local, or other regional department or

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individual such as a fire chief; fire marshal; chief of a fire prevention bureau, labor department, or health department; building official; electrical inspector; or others having statutory authority. For insurance purposes, an insurance inspection department, rating bureau, or o r other insurance insurance compan co mpany y representativ representat ivee may be the authority having having jurisdiction jurisdiction.. In many circumstances, the property owner or his or her designated agent assumes the role of the authority having jurisdiction; at government installations, the commanding officer or departmental official may be the authority having jurisdiction. A.3.2.4 Listed. The means for identifying listed equipment may vary for each organization concerned with product evaluation; some organizations do not recognize equipment as listed unless it is also labeled. The authority having jurisdiction should utilize the system employed by the listing listing organization to identify identify a listed product. A.3.3.5 Combustible Particulate Solid. Combustible particulate solids include dusts, fibers, fines, chips, chunks, flakes, or mixtures of these. A definition of this breadth is necessary because it is crucial to t o address the t he fact that there is attrition at trition of o f the t he material as it is conveyed. Pieces and particles rub against each other and collide with the walls of the duct as they travel through the system. The rubbing and collision breaks down the material and produces a mixture of pieces and much finer particles, called “dusts.” Consequently, it is expected that every conveying system produces dusts, regardless of the starting size of the material, as an inherent byproduct of the conveying process. A.3.3.14 Flame Arrester. The emerging gases are sufficiently cooled to prevent ignition on the protected side.

325, Guide to Fire Hazard Properties of A.3.3.17 Flammable Limits. See NFPA 325, Guide Flammable Liquids, Gases, and Volatile Solids. Solids . (Note: Although NFPA 325 has has been been officially withdrawn from the National the National Fire Codes®, the information is still available in NFPA's NFPA's Fire Protection Guide to Hazardous Materials.) Materials.) A.3.3.22 Isolation. Stream properties include deflagration, mass flow, ignition capability. A.3.3.23 Limiting Oxidant Concentration (LOC). Materials other than oxygen can act as oxidants. A.4.1 It should be recognized that there are other methods for preventing combustion. These include changing the process to eliminate combustible material either used or generated in the process. (Deflagration ventin venting g is not addressed in this standard; see NFPA 68, Guide 68, Guide for Venting of Deflagrations.) Deflagrations.) A.4.6 Inspection, maintenance, and operator training are necessary requirements of any explosion prevention system. Reliability of the system and its instrumentation is only as good as the inspection and periodic preventive maintenance they receive. Operator response and action to correct adverse conditions, as indicated by instrumentation or other means, is only as good as the frequency and thoroughness of training provided. A.4.6.1 Analyzers and other system instrumentation can require more frequent periodic inspection than that required for other components of the system. Inspections should be made according to the manufacturer's recommendations or as required by operating conditions and inspection history.

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A.5.1.1 Operation of a system with an oxidant concentration low enough to prevent a deflagration does not mean that incipient fires are prevented. Smoldering can occur in fibrous materials or dust layers at very low oxidant concentrations, which can ultim ately result in a fire or explosion when exposed to higher oxidant concentrations. Caution should be exercised when opening such systems to the air. air. (See Annex B for a discussion of of the control of combustible gas mixtures. Also see Annex C for limiting oxidant concentrations.) c oncentrations.) A.5.2.1 Purge gases generated by any of the acceptable methods described in this standard might not necessarily be compatible for all applications. In general, the physical and chemical properties of the t he comb co mbustibl ustiblee materials materials involv involved ed govern the t he type t ype and required purity of o f the t he purge gas needed. Chlorinated Chlorinated and fluorinated fluorinated hydrocarbons are sometimes sometimes used. Although Although these gases are more costly than carbon dioxide or nitrogen, the allowable oxygen concentration might be higher. The user is cautioned, however, that some halogenated hydrocarbons, carbon dioxide, and even nitrogen at elevated temperatures might react violently with certain dusts. Also, such gases might not be effective in providing explosion protection protect ion for certain comb co mbustibl ustiblee metal dusts, such as alumin aluminum, um, magnesium, magnesium, titanium, zirconium, thorium, and uranium. Argon, helium, and other rare gases might have to be used for inerting certain systems.

In general, personnel should not enter enclosures where the atmosphere is oxygen deficient. If it is necessary to enter such an enclosure, personnel should use self-contained breathing apparatus, preferably the positive-pressure type. Canister-type gas masks should not be used; they do not supply oxygen and do not offer any protection. The toxicity of certain purge gases should be recognized. The potential for accidental release of purge gases into normally occupied areas should be recognized and the necessary precautions taken. obta ined under the conditions specified in the Table C.1(a), A.5.2.2.1 The values were obtained Table C.1(b), Table C.1(c), C.1(c ), and Table C.2. Higher energy ignition sources, sourc es, higher higher temperatures, or higher pressures could reduce the LOC values shown. LOC values for dusts of a particular chemical composition could also differ with variations of physical properties such as particle size, shape, and surface characteristics. A particular dust could have combustion properties that differ from those shown in the tables in Annex C. Tabular data for combustion characteristics are provided as examples only. A.5.2.3.4 Under certain conditions of reducing atmospheres in the presence of sulfur compounds, pyrophoric iron sulfides could form in air-starved atmospheres. When admitting air into such an atmosphere, the iron sulfides could ignite. A typical procedure for controlling such ignition is to thoroughly wet the iron sulfide deposits with water and maintain a wetted surface until all deposits are removed and disposed of safely and properly. Another method is to maintain an inert atmosphere in the tank or vessel containing pyrophoric iron sulfides. ANSI/API 2016, Guidelines 2016, Guidelines and Procedures for Entering and Cleaning Petroleum Storage Tanks Tanks (August 2001), provides information covering the control and removal of pyrophoric iron sulfide deposits.

Rapid oxidation tends to occur when the deposits dry out. Thus, even though air is admitted slowly, nothing happens until the deposits dry out, a process that could take more time than used to admit air. A common practice in industries that deal with such deposits is to keep them wet until they can be removed to a safe location. Copyright NFPA

Iron sulfide deposits are often thick or are shielded from air by layers of nonreactive materials. When the layers are subsequently disturbed, the deposits could ignite. Furthermore, although procedures are often used to neutralize or remove such deposits before admitting air, it is often diffi difficult cult to t o remove all traces tr aces of pyrophoric material. A.5.3.2(4) The rate of application for steam inerting should be sufficient to maintain a steam concentration of at least 1.13 kg/min/2.83 m 3 (2.5 lb/min/100 ft 3). A.5.5.5 This requirement is intended to provide for a sufficient number of isolation points to facilitate maintenance, while holding the number of isolation valves to a manageable number so that accidental shutoff is minimized. A.5.5.7 Consideration should be given to providing a positive means of preventing the backflow backflow of o f purge gas into other o ther systems systems where such flow would present a hazard. A.5.7.1 The objective is to maintain operation outside of the flammable region. This can be achieved by adding either enrichment gas (natural gas or methane) or an inert gas such as nitrogen. In either case, a safety factor should be maintained between the operating condition and the closest point of the flammable region. Instrumentation should have redundancy, depending on the criticality of the operation.

system of fuel plus plus oxidant plus inert gas requires a A.5.7.2.1 As shown in Annex B, any system certain minimum concentration of oxidant for combustion. For oxidant concentrations less than the limiting oxidant concentration (LOC), no combination of fuel plus diluent can result in a flammable mixture. A.5.7.2.4 Calculation of the LOC can result in an overestimation of up to at least 2 volume percent oxygen oxygen relative t o measured values, and this potential pot ential error should be t aken into account when applying the safety margin. A.5.7.2.7.1 Products with relatively high vapor pressures can, by themselves, maintain an atmosphere above the upper flammability limit of the vapor. Where flammable atmospheres are predicted, it is common practice to use a padding gas to maintain the oxygen content at less than the LOC. Because such maintenance typically involves almost complete replacement of air, oxygen analysis of the vapor space is not generally needed. It should be ensured that padding gas capacity maintains padding under adverse conditions, such as simultaneous pump-out of several tanks connected to the same padding supply, possibly with a contraction of vapor volume caused by a sudden summer rainstorm. Such conditions might cause air to be drawn into a container to avoid underpressure damage. Also, some monomer tanks need several percent of oxygen to activate dissolved inhibitors. Such tanks might need oxygen monitoring. A.5.7.3.1 The use of enrichment gas (methane or natural gas) serves the following three purposes:

(1)

It elevates elevates the total tot al fuel concentration and can raise it to above above the upper flamm flammable able limit (UFL).

(2)

It decreases the oxidant oxidant concentration in proportion to the concentration of enrichment gas.

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(3)

It elevates elevates the LOC due to the better diluen diluentt qualities qualities of enrichmen enrichmentt gas relative relative to nitrogen in the air.

Where header systems continuously convey vapors to a combustion device such as a flare, operation above the UFL can greatly reduce the quantity of enrichment gas relative to operation below the LOC. Marine vent collecting header operation is regulated by 33 CFR 154. Nonmarine Nonmarine vent collection collection headers operated near atmospheric atmospheric pressure and not containing containing any vapor with a UFL greater than 75 percent in air, or oxygen in concentrations greater than can be derived from ambient air, can be rendered nonflammable by the addition of 25 volume percent or more of natural gas or methane. The use of oxygen analyzers to control enrichment gas flow is only practical in cases where the nitrogen-to-oxygen ratio is the same as in the air. Where a container has been partly inerted with a diluent such as nitrogen, enrichment gas should be added using flow control, since control via oxygen analyzers would otherwise add insufficient enrichment gas to provide nonflammability. The flow control system can be augmented with gas analyzers to verify correct operation during installation and for periodic performance checks. No specific specific recomm reco mmendations endations can be provi pro vided, ded, and testing t esting is necessary t o develop develop an enrichment method under the following conditions: (1)

Where system temperature tempera turess and pressures pressur es significantly significantly exceed atmospheric atmospher ic conditions

(2)

Where gases with UFL above 75 percent perce nt in air are involved involved

(3)

Where oxygen enrichment might might occur

The UFL generally increases with increased temperature and pressure; it can be sensitive to the precise gas composition and test conditions. Special procedures are needed for decomposable gases, and such procedures can involve inerting, enrichment, or deflagration isolation systems systems as described in Chapter 9. The UFL is a continuous function of oxygen concentration. The greatest UFL corresponds to pure oxygen as the oxidant and the smallest corresponds to the LOC concentration of oxidant (see Figure B.1). B.1). Systems containing high concentrations of fuel might be safely operated above the LOC, provided that they are nonflammable with respect to the actual UFL envelope. If the oxygen concentration in a system is constrained below a value whose corresponding UFL is U, a safety factor should be applied such that the fuel concentration in the system is maintained at not less than 1.7 U. This is consistent with the method in 33 CFR 154, for enrichment of marine vapor collection headers with air as the oxidant. Realistic testing is required to develop the ULF locus as a function of oxygen concentration under worst credible case operating conditions. A.6.1 See Annex B for a discussion of the control contr ol of flamm flammable able gas mixtures. mixtures. Also, see Annex D for informat information ion on calculating calculating the time required for ventilation. A.6.2.2 See NFPA 325, Guide 325, Guide to Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids. Solids. (Note: (Not e: Although NFPA 325 has been officially officially withdrawn from the

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National Fire Codes®, the information is still available in NFPA's Fire NFPA's Fire Protection Guide to Hazardous Materials.) Materials .) A.6.3.2 The combustible concentration can be reduced by recirculating the atmosphere in which it is contained through a catalytic oxidation unit where the combustible material and oxidant undergo catalytic oxidation at concentrations below the lower flammable limit. A.7.2.3 The effectiveness of spark detection and spark extinguishing systems is limited by detection of radiant energy emitted from sparks or embers in the material being conveyed and by the ability to deliver extinguishment medium (usually water) in a timely manner. For pneumatic pneumatic conveyin conveying g systems, systems, detection of sparks or embers embers is a function function of o f the t he following: following:

(1)

Pipe diameter diameter

(2)

Material-to-air ratio

(3)

Conveying Conveying velocity

(4)

Material Mater ial density and particle part icle size distribution

(5)

Radiant Radiant energy absorption characteristic of the material

Manufacturers should be consulted for the applicability of spark detection and spark extinguishing systems for specific applications. In some cases, testing could be required. A.7.3.1 Optical detectors operating in the infrared and near-infrared wavelength can be used for this technology. For information on detectors, see NFPA 72, 72, National Fire Alarm Code. Code. A.8.3.1 Experience has shown that performing maintenance operations without disarming a suppression system could result in inadvertent discharge of the suppression system. A.8.8 Halogenated hydrocarbons, such as bromochloromethane, or dry chemical agents might be used with most combustibles. The suitability of the suppressant should be determined if elevated temperatures or pressures are anticipated or if the oxidant is a material material other than air.

Water might also be used as a suppressant if it can be demonstrated to be effective. If ambient temperatures below 0°C (32°F) are expected, freeze protection should be provided. A.8.11.1 Ease of inspection should be taken into account when designing systems. A.8.11.3 The quantity of agents in containers can be checked by weighing or by using a reliable level-measuring device. A.9.1.1 It is frequently impossible to design and operate equipment without interconnecting pipes or ducts. Uses for pipes or ducts include include conveying, conveying, transferri t ransferring, ng, and ventilatin ventilating. g. Where the pipes or ducts contain flammable or combustible materials plus an oxidant, ignition can result in the communication of combustion between the interconnected equipment. Such communication of combustion can sometimes increase the violence of the deflagration, resulting in pressure piling and accelerated rates of pressure rise in the interconnected equipment from flame-jet ignition. Pressure piling can increase maximum pressure, P max, thus increasing the demands of deflagration pressure containment; and

flame-jet ignition can increase deflagration venting requirements requirements (see NFPA 68, Guide for Copyright NFPA

Venting of Deflagrations). In Deflagrations). In extreme cases, the accelerating effect of turbulent combustion through pipes or ducts plus any increased effects from pressure piling can result in detonations. A.9.1.5 See Table A.9.1.5. Table A.9.1.5 Isolation Features of Pipe and Duct Protection Systems System

Deflagration Deflagra tion Isolation

Rotary valves*

Yes

Ignition Source Isolation Note

Flow Isolation

Flame arresters Automatic fast-acting valves

Yes Yes

Yes Yes

No Yes

Flame front diverters

No

No

Yes

Flame front extinguishing systems

Yes

Yes

No

Liquid seals

Yes

Yes

No

Yes

*Rotary

valves are capable of preventing flame front passage under certain conditions but do not always prevent the passage of burning embers.

A.9.2 The acceptance of a rotary valve for use as a deflagration isolation device should consider the minimum ignition energy and the minimum ignition temperature of the dust. Additionally, the width and the length of the gap should be related to these dust characteristics. Information on testing techniques and the relationship of the factors is found in “Rotary Valves for Explosion Isolation” by G. Schuber.

The passage of a flame front through the rotary valve is not the only mechanism by which ignition can occur downstream of a rotary valve. The passage of smoldering embers through the valve might be a source of ignition on the downstream side of the valve. A.9.2.4 The use of plastics, elastomers, or other synthetic material for the full vane or as wear strips might allow the flame front to pass through the valve. The flame might pass through the valve because of a lack of mass and low specific heat that is not sufficient to cool the flame during its passage. A.9.3 Flame arresters are manufactured in several mechanical configurations, which include, but are not limi limited ted to, the following: following:

(1)

Banks of closely spaced parallel plates

(2)

Banks of small-diamete small-diameterr tubes

(3)

Wire Wire screens

(4)

Elements consisting of alternating alternat ing flat and crimped plates that are spirally wound together to produce the equivalent of small-diameter tubes

(5)

Porous or sintered metal elements elements

A.9.3.2 The ignition source might be outside the protected system, as in the case of a flame

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arrester on a tank vent. Alternatively, the ignition source might be within the system, as in the case of a flame arrester installed in a pipe that connects two or more tank vents. A.9.3.4 A deflagration flame front propagating in piping systems can transition to detonation velocities. In such situations, in-line flame arresters can be ineffective and detonation arresters should be considered. Flame arresters are reliable only where installed within the parameters for which they have been tested. Such parameters include the following:

(1)

The fuel mixtur mixturee used in the test should be the same as, or have flame flame propagation propa gation characteristics similar to, those encountered in the application.

(2)

The length of pipe between betwee n the arrester arre ster and the likely ignition ignition source sourc e should be less than or equal to the maximum length for which it was successfully tested.

(3)

The smallest smallest and largest size of a particular type of arrester should should be tested.

(4)

The arrester arre ster should be tested tes ted in the same configuration in which it will be installed, including the following: (a)

Arresting Arrest ing element

(b)

Case where it is contained

(c)

Hardware for mounting mounting the element element in its case

(d)

Gaskets or seals required

(e)

Flange Flange or other connector used to attach the arrester to the system

(f)

Materials of construction

(5)

The maxim maximum um temperature and pressure likely likely to exist exist at the arrester at the moment moment of ignition should be used.

(6)

Where appropriate for the specific specific application, application, testing with ignition ignition both upstream and downstream (relative to the gas flow direction) should be performed.

(7)

The device device should be tested teste d over the range of flow flow velocities velocities that could be encountered.

(8)

If continuous burning burning can occur at the arrester, the test procedure should include include a continuous burn test.

A.9.3.5 The functionality of a flame arrester can be destroyed if it is heated to an excessively high temperature by the combustion gases that reach it or by exposure to an external source of heat such as a flame.

The functionality-limiting temperature is dependent on the design, mass, and material of construction of the flame arrester and is unique to the design. The functionality-limiting temperature should be determined by test and should be below the autoignition temperature. A.9.3.6 If the arrester is used in a service where freezing or plugging might occur, some means of detecting the onset of plugging, such as a differential pressure switch, should be

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provided. A.9.4 See Figure Figure A.9.4.

FIGURE A.9.4 Typical Application and Design of Fast-Acting Automatic-Closing Automatic-Clo sing Valve Assembly. A.9.4.4 Fast-acting valve systems are reliable only when designed and installed within the parameters for which which they t hey have been t ested. Such parameters include, include, but are not limi limited ted to, the following:

(1)

The flame propagation propa gation characteristics charact eristics used in the placement design should be representative of the fuel mixture that provides the highest flame speed and the maximum conveying velocities to be encountered in the application.

(2)

The response respo nse time of the fast-acting fast-a cting valve system should be establi esta blished shed by testing. test ing. This response time is used in the placement design to calculate the required length of pipe between the fast-acting valve and the detector. detector .

A.9.4.9.1 Detectors that respond to radiant energy might be used, provided that the application environment does not inhibit their proper operation. Airborne dust particles, dust coating of the detector viewing window, certain gases, and the distance to the ignition source might inhibit sufficiently rapid response to the hazard. A.9.5 A flame front diverter is composed of a body and a closure device. The pressure wave that precedes the flame front opens the closure and the body diverts the flame front to the atmosphere. Some flame front diverters are equipped with an internal closure that, upon activation, creates a physical barrier to downstream flame propagation. Flame front diverters have demonstrated the ability to divert deflagration flames by directing them to the atmosphere. However, in some cases, tests have indicated that some diverters have been ineffective in completely diverting a deflagration; but, where this has occurred, the

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deflagration severity in the system has has been reduced. See Figure A.9.5.

FIGURE A.9.5 Typical Application and Design of Flame Front Front Diverters. A.9.5.9 The testing of flame front diverters should include, but not be limited to, the following:

(1)

The test fuel mixture mixture should be the same as, or have have flame flame propagation characteristics similar to, those encountered.

(2)

The length of pipe between betwee n the installed flame front diverter and the ignition source sourc e should be less than or equal to the maximum length for which the diverter was tested.

(3)

Ignition Ignition source location (upstream, downstream, or both locations) should should be tested in the same configuration as the protection application.

(4)

For upstream ignition, ignition, the diverter should be tested over the range of flow flow velocities velocities that could be encountered at the time of ignition or that might develop as a result of ignition.

(5)

Installation and maintenance should be as follows: (a)

Flame front divert diverters ers should be installed and maintained maintained according to manufacturer's manufacturer's instructions. instructions.

(b)

Flame front diverters should be inspected periodically, based on facility facility experience, and after each operation.

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(c)

Inspection should determine determine whether any damage that could affect affect the performance performance of the t he device has occurred.

(d)

Damaged components components should should be repaired or replaced. replaced.

A.9.7 For most systems, API Recommended Practice 521, Guide for Pressure-Relieving and Depressuring Systems, should Systems, should be used when designing liquid seals. For systems where the oxidant/flammable gas might approach a stoichiometric mixture, tests should be performed to determine determine the design of liquid liquid seals. A.9.7.3 Where the inlet gas is a combustible mixture, additional precautions should be taken to prevent overheating of inlet piping within the liquid seal device by a continuous fire in the seal enclosure. A.10.1.5 Pressure piling and flame-jet ignition can significantly increase deflagration pressures in attached atta ched equipment. equipment. Techniques Techniques such as isolation isolation or o r venting venting should be considered. A.10.2.1 Deflagration pressure containment is not adequate for detonable systems because the maximum maximum pressure pressur e rise is much greater great er than the factors facto rs established in in 10.3.3.1 10.3.3. 1 through 10.3.3.4. It should be recognized that some systems might be capable of deflagration or detonation. For example, systems containing a substantial proportion of hydrogen are prone to detonation, as are systems containing acetylene or acetylenic compounds. Saturated organic compounds such as propane, ethane, and alcohols generally do not detonate in vessels but might do so in pipework. Internals in equipment can promote the transition from deflagration to detonation. A.10.2.2 When two vessels connected by a large-diameter pipe both contain a combustible mixture, a deflagration in one vessel can precompress the unburned mixture in the other vessel. The maximum deflagration pressure that can be developed in the second vessel might be substantially substantially greater than would normally normally happen in a single single vessel. (See vessel. (See W. Bartknecht, Explosions: Course, Prevention, Protection, pp. 18–23.) A.10.2.3 Only limited information is available for deflagration containment of systems with initial gauge pressures exceeding 2 bar (30 psi). Increased initial pressure might increase the potential for detonation. For this reason it is recommended that, f or or systems that might operate at an initial gauge pressure of 2 bar (30 psi) or higher, deflagration pressure containment should be used only where applicable test data are available. The testing should be carefully designed designed because the detonation potential of a system is affected by vessel dimensions. A.10.3.3 For vessels fabricated of low-carbon steel and low-alloy stainless steel, Fu equals

approximately 4.0 and F y equals approximately 2.0. The formulas are based on a paper by Noronha et al., “Deflagration Pressure Containment Containment for Vessel Safety Containment Containment for Vessel Safety Design, Plant/Operations Progress.” A.10.3.3.1 The maximum deflagration pressures for several dusts can be found in Annex D of NFPA 68, Guide 68, Guide for Venting of Deflagrations. Deflagrations. A.10.3.5 The maximum initial pressure depends on the origin of the pressure. In some cases,

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the maximum initial pressure is determined by the setting of a relief device on the system. In such cases, the maximum initial pressure is the sum of the relief device set pressure and the relief device accumulation pressure. Overpressure due to boiling of the vessel contents (for example, from external fire exposure) might raise the concentration of fuel in the vapor phase above its upper flammable limit and does not constitute a deflagration hazard.

Annex B Control of Flammable Gas Mixtures by Oxidant Concentration Reduction and Combustible Concentration Reduction T his his annex is not a part of the requirements of this NFPA document but is included for informational purposes only. B.1 General.

As covered in Chapters Chapter s 5 and 6, a flammabl flammablee gas/oxidant mixtur mixturee might might be controlled by reducing the concentration of oxidant or by adding an inert constituent to the mixture. Both processes can be explained explained most easily easily by referring to a flamm flammabi abili lity ty diagram. Figure Figure B.1 shows a typical flammability diagram that represents a mixture of a combustible gas, an inert gas, nitrogen, and an oxidant, oxygen, at a given temperature and pressure. A mixture of air (79 percent N 2 and 21 percent O 2, by volume) and combustible gas is represented by the line formed by points DABE points DABE . A given mixture of the combustible gas and air, whether ignitible or not, is specified by a point on this line. Point A indicates the upper flammable limit of this mixture, and point B represents its lower flammable limit.

FIGURE B.1 Typical Flammability Diagram.

Any point within the area bounded by FBCAGF is is in the flammable range and can be ignited. Copyright NFPA

Any point outside this area represents a mixture that cannot be ignited. Point C represents C represents the limiting oxidant concentration to prevent ignition; any mixture containing less oxygen cannot be ignited. (See Annex C .) .) Any mixture of oxygen and combustible gas alone (that is without nitrogen) is represented by the left side of the triangle. Any mixture of nitrogen and combustible gas alone (that is without oxygen) is represented by the right side of the triangle. B.2 Effect of Pressure and Temperature.

As shown in in Figure B.2, pressure pressur e and temperature temperat ure can have an effect on the flammabil flammability ity diagram. An increase in pressure results in an increase in the upper flammable limit and a decrease in the limiting oxidant concentration points C , C' , and C , to prevent prevent ignition. There is a slight decrease on the lower flammable limit, but the effect is not as pronounced as that of the upper limit.

FIGURE B.2 Effect of Pressure on Flammability Diagram.

An increase in temperature has a similar effect on the flammability diagram. The exact effects on a system, produced by changes in pressure or temperature, should be determined for each system. B.3 Effect of Inert Diluents.

The addition of an inert diluent to a mixture of combustible material and oxidant affects the lower and upper flammable flammable limi limits ts and the limiting limiting oxidant oxidant concentrat concent ration. ion. Figure B.3 illustrates the effect of some typical diluents on the flammability limits of methane. Figure B.3 shows that nitrogen is more effective than helium and that carbon dioxide is more effective than nitrogen.

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FIGURE B.3 Limits of Flammability of Methane-Inert Gas-Air Mixtures at 25°C (77°F) and Atmospheric Pressure. (Source: J. F. Coward and G. W. Jones, “Limits of Flammability of Gases and Vapors.”) B.4 Oxidant Concentration Reduction.

In Figure B.1, point point X represents X represents an arbitrary mixture of flammable gas, oxygen, and nitrogen that lies well within the flammable range. If the composition of the mixture is to be changed so that it lies outside the flammable range, one method that can be used is to reduce the concentration of oxidant. As the concentration of oxygen decreases, the concentration of nitrogen increases. Point X , in effect, moves toward the inert gas apex. B.5 Combustible Concentration Reduction.

In Figure B.1, with point X in X in the flammable range, the composition of the mixture might be altered by reducing the concentration of flammable gas. In simpler terms, point X moves X moves away from the flammable gas apex and eventually drops below the lower flammability flamm ability line Copyright NFPA

FBC . B.6 Mixtures of Gases.

Where mixtures of two or more flammable gases are encountered, the limits of flammability of the mixture can often be reliably predicted by using the following formulas suggested by Le Chatelier:

(B.1)

(B.2)

where: P1 . . . Pn = volume fractions of components 1, 2, 3, . . . , n of the mixture LFL 1

LFLn = lower flammabl flammablee limits limits of components 1, 2, 3,

UFL 1

UFLn = upper flamm flammable able limits limits of components 1, 2, 3,

, n of the mixture , n of the mixture

Annex C Limiting Oxidant Concentrations T his his annex is not a part of the requirements of this NFPA document but is included for informational purposes only. C.1 General.

The Table C.1(a), C.1(a ), Table C.1(b), and Table C.1(c) provide values for limi limiting ting oxidant concentration concentr ation (LOC) using nitrogen, nitroge n, carbon dioxide, dioxide, and inert dust as the diluent. diluent. Table C.1(a) provides pro vides LOC values values for for flammable flammable gases, and Table C.1(b) and Table C.1(c), provide data for combustibl combustiblee dust suspensions. suspensions. Table C.1(a) Limiting Limitin g Oxidant Concentrations for Flammable Gases When Using Nitrogen or Carbon Dioxide as Diluents Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) N2/Air 12.0

CO2/Air 14.5

Reference 1

Ethane Propane

11.0 11.5

13.5 14.5

1 1

n-Butane n-Butyl acetate

12.0 9.0

14.5 —

1 9

Isobutane n-Pentane

12.0 12.0

15.0 14.5

1 1

Gas/Vapor Methane

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Table C.1(a) Limiting Limitin g Oxidant Concentrations for Flammable Gases When Using Nitrogen or Carbon Dioxide as Diluents Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) N2/Air 12.0

CO2/Air 14.5

Reference 2

n-Hexane

12.0

14.5

1

n-Heptane Ethanol Ethan ol

11.5 8.7

14.5 —

2 9

Ethylene Propylene

10.0 11.5

11.5 14.0

1 1

1-Butene Isobutylene Isobutylene

11.5 12.0

14.0 15.0

1 4

Butadiene 3-Methyl-1 butene Benzene

10.5 11.5

13.0 14.0

1 4

11.4

14.0

1, 7

Toluene Styrene

9.5 9.0

— —

7, 9 7

Ethylbenzene

9.0

—

7

Vinyltoluene

9.0

—

7

Divinylbenzene Diethylbenzene

8.5 8.5

— —

7 7

Cyclopropane Cyclopropane Gasoline

11.5

14.0

1

(73/100)

12.0

15.0

2

(100/130)

12.0

15.0

2

(115/145) Kerosene

12.0 10.0 (150°C)

14.5 13.0 (150°C)

2 5

JP-1 fuel JP-3 fuel JP-4 fuel

10.5 (150°C) 12.0 11.5

14.0 (150°C) 14.5 14.5

2 2 2

12.0 14.0

14.5 —

1 3

12.0 (100°C)

—

3

19.0 (30°C) 17.0 (100°C)

—

3

— —

3 3

14.0

— —

3 3

9.0 (100°C)

—

3

11.5

14.0

4

Gas/Vapor Isopentane

Natural gas (Pittsburgh) n-Butyl chloride Methylene chloride

Ethylene dichloride

1,1,1-trichloroethane Trichloroethylene Acetone

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13.0 11.5 (100°C)

Table C.1(a) Limiting Limitin g Oxidant Concentrations for Flammable Gases When Using Nitrogen or Carbon Dioxide as Diluents Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) N2/Air NA

CO2/Air 16.5 (150°C)

Reference 4

Carbon disulfide

5.0

7.5

4

Carbon monoxide Ethanol Ethan ol

5.5 10.5

5.5 13.0

4 4

9.5 (150°C) 10.5

— 13.0

4 4

Hydrogen Hydrogen Hydrogen Hydrogen sulfide

5.0 7.5

5.2 11.5

4 4

Isobutyl Isobutyl acetate Isobutyl Isobutyl alcohol

9.1 9.1

— —

9 9

Isobutyl Isobutyl formate Isopropyl Isopropyl acetate

12.5 8.8

15.0 —

4 9

Isopropyl Isopropyl alcohol Methanol

9.5 10.0

— 12.0

10 4

Methyl acetate Propylene oxide

11.0 7.8

13.5 —

4 8

Methyl ether

10.5

13.0

4

Methyl formate

10.0

12.5

4

Methyl ethyl ketone n-Propyl acetate

11.0 10.1

13.5 —

4 10

n-Propyl alcohol UDMH (dimethylhydrazine) Vinyl chloride

8.6 7.0

— —

9 6

13.4

—

7

Gas/Vapor n-Butanol

2-Ethyl butanol Ethyl ether

Vinylidiene chloride 15.0 — 7 Notes: 1. See 5.7.2 for the required oxygen oxygen level level in equipment. 2. Data were determined by laboratory experiment conducted at atmospheric temperature and pressure. Vapor-air-inert gas samples were placed in explosion tubes and ignited by electric spark or pilot flame. References for Table C.1(a). 1. J. F. Coward and G. W. Jones (1952). 2. G. W. Jones, M. G. Zabetakis, J. K. Richmond, G. S. Scott, and A. L. Furno (1954). 3. J. M. Kuchta, A. L. Furno, A. Bartkowiak, and G. H. Martindill (1968). 4. M. G. Zabetakis (1965). 5. M. G. Zabetakis and B. H. Rosen (1957). 6. Unpublished data, U.S. Bureau of Mines. 7. Unpublished data, Dow Chemical Co. 8. U.S. Bureau of Mines. 9. L.G. Britton (2002). 10. Un Un ublished Dow Chemical Co. 2002.

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Table C.1(a) Limiting Limitin g Oxidant Concentrations for Flammable Gases When Using Nitrogen or Carbon Dioxide as Diluents Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) Gas/Vapor

N2/Air

CO2/Air

Reference

10. Unpublished, Dow Chemical Co. 2002.

Table C.1(b) Limiting Limitin g Oxidant Concentrations for Combustible Dust Suspensions When Using Nitrogen or Carbon Dioxide as Diluents Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) Dust Agricultural

N2/Air

Cof fee fee Cornstarch Dextrin Soy flour Starch Sucrose

CO2/Air

17 11 11

10

14 15 12 14

Chemical Ethylene diamine tetra-acetic acid

13

Isatoic anhydride Methionine

13 15

Ortazol Phenothiazine

19 17

Phosphorus pentasulfide

12

Salicylic acid Sodium lignosulfate Stearic acid & metal stearates

15

17

10.6

17 13

Carbonaceous Charcoal Coal, bituminous

17 17

Coal, sub-bituminous Lignite

15 15

Metal Aluminum

2

Antimony Chromium

16 14

Iron Magnesium

10 0

Manganese

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0

14

Table C.1(b) Limiting Limitin g Oxidant Concentrations for Combustible Dust Suspensions When Using Nitrogen or Carbon Dioxide as Diluents Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) Dust

N2/Air

CO2/Air

Paper

13

Pitch Sewage Sewage sludge

11 14

Sulfur Wood flour

12 16

Plastics Ingredients Azelaic acid

14

Bisphenol A Casein, rennet

12 17

Hexamethylene Hexamethylene tetramine Isophthalic acid

13

14 14

Paraformaldehyde

8

12

Pentaerythritol Phthalic Phthali c anhydride

13

14 14

Terephthalic Terephthal ic acid Plastics — Special Sp ecial Resins Coumaroneindene resin

15 14

Lignin Phenol, chlorinated Pinewood Pinewood residue

17 16 13

Rosin, DK Rubber, Rubber, hard

14 15

Shellac

14

Sodium resinate

13

Plastics — Thermoplastic Resins Acetal

11

Acrylonitrile Butadienestyrene Carboxymethyl Carboxymethyl cellulose Cellulose acetate Cellulose triacetate

14

13 13 16 9

11 12

Cellulose acetate butyrate

14

Ethyl cellulose

11

Methyl cellulose Methyl methacrylate

13 11

Nylon Nylon

13

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Table C.1(b) Limiting Limitin g Oxidant Concentrations for Combustible Dust Suspensions When Using Nitrogen or Carbon Dioxide as Diluents Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) N2/Air

Dust

Polycarbonate Polyethylene

CO2/Air 15

Polystyrene Polyvinyl Polyvinyl acetate Polyvinyl Polyvinyl butyrate Plastics — Thermosetting Resins Allyl alcohol

12 14 17 14 13

Dimethyl isophthalate isophthala te Dimethyl terephthalate terephtha late

13 12

Epoxy Melamine formaldehyde

12 15

Polyethylene Polyethylene terephthalate terephthal ate Urea formaldehyde

13 16

Notes: 1. Data in this table were obtained by laboratory tests conducted at room temperature and pressure, using a 24-watt continuous-spark ignition source and were reported in U.S. Bureau of Mines, Report of Investigation 6543. 2. Where nitrogen is used as the diluent and no data are listed in the table, the following equation should be used to calculate the oxygen value for carbonaceous dusts:

(C.1) where: On = limiting oxygen concentration for dilution by nitrogen (N) Oc = limiting oxygen concentration for dilution by carbon dioxide(CO 2) 3. See 5.7.2 for the required oxygen oxygen level level in equipment. 4. Data on the use of dry powders or water as inerting materials and on the effects of inerting on pressure development in a closed vessel are given in U.S. Bureau of Mines, Reports of Investigations In vestigations 6549, 6561, and 6811. 5. The values in this table can differ f rom rom those in Table C.1(c) because because of differences differences in test methods and dust characteristics, such as particle size, and other factors.

Table C.1(c) Limitin Limiting g Oxidant Concentrations for Combustible Dust Suspensions When Using Nitrogen as a Diluent

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Above Which Deflagration Can Take Place) N2/Air

Dust

CO2/Air

Table C.1(c) Limitin Limiting g Oxidant Concentrations for Combustible Dust Suspensions When Using Nitrogen as a Diluent Dust

Median Particle Diameter by Mass (

m)

Limiting Oxidant Concentration (Volume % O2 Above Which Deflagration Can Take Place) N2/Air

Cellulosic Materials Cellulose

22

9

Cellulose

51

11

27

10

Pea flour Corn starch

25 17

15 9

Waste from malted barley Rye flour

25 29

11 13

Starch derivative Wheat flour

24 60

14 11

Coals Brown coal

42

12

Brown coal Brown coal

63 66

12 12

Brown coal briquette dust Bituminous coal

51 17

15 14

<63

10

Rubber powder

95

11

Polyacrylonitrile Polyacrylonitrile Polyethylene, Polyethylene, h.p.

26 26

10 10

<10

9

<10

12

<63

13

Benzoyl Benzoyl peroxide Bisphenol A

59 34

10 9

Cadmium laurate

<63

14

Cadmium stearate

<63

12

Calcium stearate Methyl cellulose

<63 70

12 10

27

9

Wood flour Food and Feed

Plastics, Resins, Rubber Resin

Pharmaceuticals, Pesticides Amino phenazone Methionine Methionine Intermediate Products, Additives A dditives Barium stearate

Dimethyl terephthalate terephtha late

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Table C.1(c) Limitin Limiting g Oxidant Concentrations for Combustible Dust Suspensions When Using Nitrogen as a Diluent Dust

Median Particle Diameter by Mass (

Limiting Oxidant Concentration (Volume % O2 Above Which

m)

Deflagration Can Take Place) N2/Air

Ferrocene Bistrimethylsilyl-urea Naphthalic Naphthal ic acid a cid anhydride a nhydride

95 65

7 9

16

12

2-Naphthol Paraformaldehyde Pentaerythritol

<30 23

9 6

<10

11

22

5

Calcium/ aluminum alloy

22

6

Ferrosilicon magnesium alloy Ferrosilicon alloy

17 21

7 12

Magnesium alloy Other Inorganic Products

21

3

<10 13 16

12 12 12

43

12

Metals, Alloys Aluminum

Soot Soot Soot Others Bentonite derivative

Source: R. Source: R. K. Eckhoff, Dust Eckhoff, Dust Explosions in the Process Industries, Industries, 1991. Note: The Th e data dat a came from 1-m3 and 20-L chambers using strong chemical igniters.

C.2 General.

Table C.2 provides data on the concentration of inert dust required to inert selected combustible dusts. Table C.2 Inerting of Dust Clouds by Mixing the Combustible Dust with Inert Dust Combustible Combustible Dust Dust

Inert Dust

Median Particle Size by Mass (

m )

Methyl cellulose

70

Organic pigment

<10

Bituminous coal

20

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Type of Dust


CaSO4 NH4H2PO4

<15

Minimum Mass % Inert of Total Mass Required for Inerting 70

29

65

14

65

m )

Table C.2 Inerting of Dust Clouds by Mixing the Combustible Dust with Inert Dust Combustible Combustible Dust Dust

Inert Dust


Type of Dust

Median Particle Size by Mass

m )

Bituminous coal

20

Sugar

30

(

NaHCO3 NaHCO3

m )

Minimum Mass % Inert of Total Mass Required for Inerting

35

65

35

50

Source: R. Source: R. K. Eckhoff, Dust Eckhoff, Dust Explosions in the Process Industries, Industries, 1991. 3 Note: Data were obtained from tests conducted in 1-m Standard ISO (1985) vessel with a 10-kJ chemical igniter.

Annex D Ventilation Calculations T his his annex is not a part of the requirements of this NFPA document but is included for informational purposes only. D.1 Time Required for Ventilation.

An estimate of the time required to reduce the concentration of a flammable gas to a safe limit by purging with fresh air can be calculated using the method that follows. For an enclosed volume, V , the change in concentration, dC , over a given time, dt , using a fixed flow rate of fresh air, Q, is given by Equation D.1: (D.1)

By rearranging,

(D.2)

where: C 0 = initial concentration of gas t = = time required to reach the desired concentration Integrating Equation D.2 yields the following:

(D.3)

Equation D.3 assumes perfect mixing. Because this is not the case in actual practice, a correction factor, K , should be introduced as follows:

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(D.4)

In perfect mixing, K equals K equals 1.0. Table D.1 lists values values of K for K for certain conditions. Few data exist on defining the degree of mixing. Most authorities recommend a K-value K-value of not greater than 0.25. Consider the problem of reducing the gasoline vapor concentration of an enclosure of 28 m 3 (1000 ft 3), using a 56 m3/min (2000 ft 3/min) ventilation rate, from 20 volume percent to the following: (1)

The upper flammable flammable limit, limit, or 7.6 percent perce nt

(2)

The lower flammable flammable limit, limit, or 1.4 percent

(3)

Twenty-five percent perce nt of the lower flammable flammable limit, limit, or 0.35 percent perce nt Table D.1 Mixing Efficiency for Various Ventilation Arrangements Efficiency (K) Values

Method of Supply No Positive Supply

Single Exhaust Opening

Multiple Exhaust Openings

0.2 0.2

0.3 0.4

Grilles and registers Diffusers Diffusers

0.3 0.5

0.5 0.7

Perforated ceiling

0.8

0.9

Infiltration through cracks Infiltration through open doors or windows Forced Air Ai r Supply S upply

The difference between K between K = = 1.0 (perfect mixing) and K = K = 0.2 in calculating the time needed to reduce the concentration to the levels specified can be shown using Equation D.3 as follows:

(D.5)

(D.6)

For K For K = = 1, t = = 0.49 min. For K = = 2.5 min. K = 0.2, t =

(D.7)

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(D.8)

For K For K = = 1, t = = 1.33 min. For K = K = 0.2, t = = 6.65 min.

(D.9)

(D.10)

For K For K = = 1, t = = 2 min. For K For K = = 0.2, t = = 10 min. D.2 Number of Air Changes Required for Inerting.

The calculation metho method d described in in Section D.1 provides a solution expressed directly in in terms of time. To develop a solution in terms of required number of air changes, the equation is written as follows:

(D.11)

where N = N = the required number of air changes. Equation D.11 can be rewritten as follows:

(D.12)

Using the example in Section Sect ion D.1, the number of air changes required to reach the upper flammable limit, 7.6 percent, at K = K = 0.2, is as follows:

(D.13)

(D.14)

Because the airflow rate is 56 m3/min (2000 ft 3/min) and the volume of the enclosure is 28 m3 (1000 ft 3), a complete air change takes 0.5 minute. Equation D.14 indicates that 4.8 air changes are needed. This translates to a required time of 2.4 minutes, or exactly that calculated calculated in Section D.1. D.3 Buildup of Combustible Concentration in Enclosed Area.

If a constant source of a flammable gas, such as a leak, is introduced into an enclosed volume, Equation D.12 should be modified as follows:

(D.15)

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where: C = C = concentration G = release rate [m3/min (ft3/min)] Q = airflow rate (m3/min) [ft3/min] K = K = mixing efficiency factor N = N = number of theoretical air changes As an example, consider a leak of 2.8 m 3/min (100 ft 3/min) of a 15 percent flammable gas/air mixture in a room of 28 m3 (1000 ft 3). How long would it take to reach a concentration of 5 percent throughout the enclosure, assuming assuming a mixi mixing ng coeff coe ffici icient, ent, K , equal to 0.2? Thus, C = C = 0.05 G = 15 ft3/min (100 × 0.15) Q = 85 ft3/min (100 – 15) K = K = 0.2 Equation D.15 can be rewritten into a more convenient logarithmic form as follows:

(D.16)

Because the volume is 100 ft 3/min and the leak is at 1000 ft 3,

(D.17)

A concentration of 5 percent is reached in 16.7 minutes. Equation D.12 and Equation D.15 can be plotted plott ed as shown in Figure D.3(a) D.3(a ) and Figure D.3(b).

FIGURE D.3(a) Combustible Decay Curve. General Ventilati Ventilation: on: Instantaneo Instantaneous us Release.

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FIGURE D.3(b) Combustible Buildup Curve. Curve. General Ventilation: Ventilation: Continuous Release.

With respect to Figure D.3(b), which illustr illustrate atess a continuous release re lease in an enclosed volume, volume, once a continuous release begins, the combustible concentration increases rapidly until three air changes occur. After three air changes, the bracketed term in Equation D.15 approaches unity and concentration does not change much. Thus, steady-state concentration is independent of air-change rate and actually depends on the volumetric flow of fresh air. For design purposes, it is best to specify in terms of cubic m eters per minute (cubic feet per minute) and avoid specifying in terms of air changes per hour. Although general ventilation is helpful in removing airborne combustibles, better control can be achieved in many cases by supplemen supplementing ting general ventilation with local ventilation. ventilation. Local ventilation can be used when the source of emission can be predicted. For example, local ventilation rather than general ventilation is recommended in the following situations: (1)

The operato oper atorr or ignition sources sour ces might might be very close to the point of flammable flammable release.

(2)

The flammable flammable escape rate rat e is uncertain. uncert ain.

(3)

Local ventilation ventilation is used to control combustibl combustiblee dusts.

Local exhaust ventilation captures the combustible at its source, and a properly designed system can achieve almost 100 percent effectiveness, provided that the local exhaust pickup can be placed close to the point of release.

Annex E Purging Methods T his his annex is not a part of the requirements of this NFPA document but is included for informational purposes only. E.1 General.

Any of several methods might be used to ensure the formation and maintenance of a noncombustible atmosphere in an enclosure to be protected. These include “batch” methods for one-time or occasional use, as in purging equipment during shutdown, and “continuous” methods intended to ensure safe conditions during normal operations. The following is an Copyright NFPA

outline of various purging methods. E.2 Purging Methods. E.2.1 Batch Purging. This method includes siphon, vacuum, pressure, and venting to atmosphere. E.2.2 Continuous Purging. This method includes fixed-rate application and variable-rate or demand application. E.2.3 Siphon Purging. In this method, equipment might be purged by filling with liquid and introducing purge gas into the vapor space to replace the liquid as it is drained from the enclosure. The volume of purge gas required is equal to the volume of the vessel, and the rate of application can be made to correspond to the rate of draining. E.2.4 Vacuum Purging. In this method, equipment that normally operates at reduced pressure, or in which it is practical to develop develop reduced pressure, might might be purged during shutdown by breaking the vacuum with purge gas. If the initial pressure is not low enough to ensure the desired low oxidant concentration, it might be necessary to re-evacuate and repeat the process. The amount of purge gas required is determined by the number of applications required to develop the desired oxidant concentration. Where two or more containers or tanks are joined by a manifold and should be purged as a group, the vapor content of each container or tank should be checked to determine that complete purging has been accomplished. E.2.5 Pressure Purging. In this method, enclosures might be purged by increasing the pressure within within the t he enclosure by introducing introducing purge gas under pressure and, after the gas has diffused, venting the enclosure to the atmosphere. More than one pressure cycle might be necessary to reduce the oxidant content to the desired percentage. Where two or more containers or tanks are joined by a manifold and should be purged as a group, the vapor content of each container or tank should be checked to determine that the desired purging has been accomplished. Where a container filled with combustible material is to be emptied and then purged, purge gas might be applied to the vapor space at a pressure consistent with equipment design limitations, thus accomplishing both the emptying of the vessel and the purging purging of the t he vapor space in the same process. E.2.6 Sweep-Through Purging. This method involves introducing a purge gas into the equipment at one opening and letting the enclosure content escape to the atmosphere through another opening, thus sweeping out residual vapor. The quantity of purge gas required depends on the physical arrangement. A pipeline can be effectively purged with only a little more than one volume of purge gas if the gas can be introduced at one end and the mixture can be released at the other. However, vessels require quantities of purge purge gas much in excess of their volume.

If the system is complex, involving side branches through which circulation cannot be established, the sweep-through purging method might be impractical, and pressure or vacuum purging might be more appropriate. The relationship between the number of volumes of purge gas circulated and the reduction in concentration of the critical component in original tank contents, assuming complete mixing, Copyright NFPA

is shown on the graph in Figure E.2.6.

FIGURE E.2.6 Dilution Ratio — Purging at Atmospheric Pressure Pressure (Complete Mixing Assumed).

The following points should be noted: (1)

The total tot al quantity required might might be less than that for for a series of steps of pressure purging. purging.

(2)

Four to five volumes of purge gas are sufficient sufficient to almost almost completely displace the original mixture, assuming complete mixing.

E.2.7 Fixed-Rate Purging. This method involves the continuous introduction of purge gas into the enclosure at a constant rate, which should be sufficient to supply the peak requirement in order that complete protection is provided, and a corresponding release of purge gas and a nd whatever gas, mist, mist, or dust has been picked up in the equipment. equipment.

The following information regarding the fixed-rate purging method should be noted: (1)

The advantages advantage s are simplicity, simplicity, lack of dependence on devices such as pressure pressur e regulators, and possible reduced maintenance.

(2)

The disadvantages disadvantages are as follows: follows: (a)

Continuous Continuous loss loss of product product where the space contains contains a volatil volatilee liquid, liquid, due to constant “sweeping” of the vapor space by the purge gas

(b) Increased total quantity quantity of purge gas, since since it it is suppli supplied ed regardless of whether it is needed (c)

Possible disposal problems (toxic (to xic and other ot her effects) for the mixture mixture continuously released

Figure E.2.7 shows a method of flow control that can be used with fixed-rate purging. Copyright NFPA

FIGURE E.2.7 Method of Flow Control for Use with Fixed-Rate Purging. E.2.8 Variable-Rate or Demand Purging. This method involves the introduction of purge gas into an enclosure at a variable rate that is dependent on demand and is usually based on maintaining within the protected enclosure an arbitrarily selected pressure slightly above that of the surrounding atmosphere. Peak supply rate should be computed as described under “Calculation of Peak Purge Gas Rates,” which follows.

The following information regarding the variable-rate or demand purging should be noted: (1)

The advantages advantage s are that purge gas is supplied only when actually act ually needed and that it it is possible, possible, when desirable, desirable, to completely completely prevent influ influx x of o f air.

(2)

A disadvantage disadvantage is that operation depends on the functionin functioning g of pressure control valves that operate at sometimes very low pressure differentials, which are sometimes difficult to maintain.

Figure E.2.8(a) shows a method of flow control that can be used with variable-rate purging. Figure E.2.8(b) shows an alternative method that is applicable where the purge gas requirement during out-pumping is a large part of the peak demand.

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FIGURE E.2.8(a) Method of Flow Flow Control for Use with Variable-Rate Variable-Rate Purging.

FIGURE E.2.8(b) Alternative Method of Flow Flow Control for Use with Variable-Rate Variable-Rate Purging. E.3 Calculation of Peak Purge Gas Rates.

Peak demand demand is described described in Section 5.6 as the total t otal expected system requirements. requirements. For any one element of the system, the peak demand is controlled by factors such as the following: (1)

Maximum Maximum withdrawal rate rat e

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(2)

Temperature change

(3)

Leaks

(4)

Rapid atmospheric atmospheric pressure changes

Cooling of the contents of a vessel containing a vapor or hot liquid presents a special and frequent case of vacuum purging. Condensation of vapor to a liquid or reduction in pressure of the gas phase can rapidly produce partial vacuum, which could result in the following: (1)

Imposition Imposition of excessive excessive stresses on equipment equipment or collapse collapse of the vessel

(2)

Sucking in of air from joints that might might not leak under internal pressure pressur e

(3)

Creation Creat ion of a need for high supply rates rat es of inert gas

Every situation should be treated individually. The peak supply rate should be computed for each case, with consideration given to cooling rate, vessel size, and configuration, which determine the rate of condensation. If neither the reducing valve nor the source gas can be relied on to supply the amount of inert gas required to prevent reduction of pressure below atmospheric, the vessel might have to be designed for full vacuum. For a vessel that contains a liquid, the purge gas demand from liquid withdrawal, change of liquid composition from mixing, or increasing solubility of purge gas in the liquid is the greater of one of the following: (1)

The volume equivalent of the capacity capac ity of the largest pump pump that can withdraw liquid

(2)

The maximum maximum possible gravity outflow out flow rate rat e

Where two tanks are manifolded together so that one can flow by gravity into the other, a vapor space interconnection is sometimes used to reduce the required purge gas supply from outside sources. For outdoor tanks operating at or near atmospheric pressure, the maximum demand from temperature change occurs in outdoor tanks operating at near atmospheric pressure as a result of sudden cooling by a summer thunderstorm. The rate of purge gas supply necessary to prevent vessel pressure falling significantly below atmospheric pressure can be calculated as follows: (1)

For tanks over 3.028 million million L (800,000 (800,00 0 gal) capacity, capacit y, 0.056 m 3 (2 ft3) of purge gas per hour for each square foot of total t otal shell shell and roof area

(2)

For smaller smaller tanks, 0.028 m3 (1 ft3) purge gas per hour for each 151 L (40 gal) of tank capacity, or the rate corresponding to a mean rate of the change of the vapor space temperature of 38°C (100°F) per hour

See API Standard 2000 , Venting Atmospheric and Low-Pressure Storage Tanks Nonrefrigerated and Refrigerated, for Refrigerated, for further information on the calculation of rate of purge gas supply. The rates for temperature change and liquid withdrawal should be added unless a special Copyright NFPA

circumstance exists that prevents them from occurring simultaneously. In some equipment, such as pulverizers, the rate of purge gas supply necessary to exclude air might be dominated by leakage, and temperature change can be ignored.

Annex F Informational References F.1 Referenced Publications.

The following documents or portions thereof are referenced within this standard for informational purposes only and are thus not part of the requirements of this document unless also listed listed in Chapter 2. F.1.1 NFPA Publications. National Fire Protection Pro tection Association, Association, 1 Batterymarch Park, P.O. Box 9101, Quincy, MA 02269-9101.

NFPA 51B, Standard 51B, Standard for Fire Prevention During Welding, Cutting, and Other Hot Work , 1999 edition. NFPA 68, Guide 68, Guide for Venting of Deflagrations, Deflagrations, 2002 edition. NFPA 72 ®, National Fire Alarm Code®, 2002 edition. NFPA 326, Standard 326, Standard for the Safeguarding of Tanks and Containers for Entry , Cleaning, or Repair , 1999 edition. Fire Protection Guide to t o Hazardous Materials, Materials , 1997 edition. F.1.2 Other Publications. F.1.2.1 API Publications. American Petroleum Institute, 1220 L Street, NW, Washington, DC 20005.

API RP 521, Guide 521, Guide for Pressure-Relieving and Depressuring Systems, Systems, Third Edition, 1990. API STD 2000, Venting 2000, Venting Atmospheric and Low-Pressure Storage Tanks Nonrefrigerated and Refrigerated , Fourth Edition, 1992. API 2016, Guidlelines 2016, Guidlelines and Procedures for Entering and Cleaning Petroleum Storage Tanks, Tanks, 2001. F.1.2.2 USBM Publications. U.S. Bureau of Mines, Columbia Plaza, 2401 E Street, NW, Washington, DC 20241.

Nagy, J., H. G. Dorsett, Dorsett , Jr., and M. Jacobson, Preventing Jacobson, Preventing Ignition of Dust Dispersions by Inerting , Report of Investigations 6543, 1964. Nagy, J., A. R. Cooper, and J. M. Stupar, Pressure Development in Laboratory Dust Explosions, Explosions, Report of Investigations 6561, 1964. Nagy, J. and D. J. Surincik, Surincik, Thermal Thermal Phenomena During Ignition of a Heated Dust Dispersion, Dispersion, Report of Investigation 6811, 1966. F.1.2.3 U.S. Government Publications. U.S. Government Printing Office, Washington, DC Copyright NFPA

20402. Title 33, Code 33, Code of Federal Regulations, Part Regulations, Part 154. F.2 Informational References.

The following documents or portions thereof are listed here as informational resources only. They are not a part of the requirements of this document. Bartknecht, W., Explosions: W., Explosions: Course, Prevention, Protection, Protection, Springer-Verlag, Springer-Verlag, Heidelberg, Heidelberg, 1981. L. G. Britton, “Using Heats of Oxidation to Evaluate Flammability Hazards,” March 2002, Process Safety Progress. Progress. Coward, J. F., and G. W. Jones, “Limits of Flammability of Gases and Vapors,” Bulletin 503, U.S. Bureau of Mines, Pittsburgh, PA, 1952. Eckhoff, R. K., Dust K., Dust Explosions in the Process Industries, Industries, Butterwort h-Heineman h-Heinemann, n, Oxford, England, 1991. Jones, G. W., M. G. Zabetakis, J. K. Richmond, G. S. Scott, and A. L. Furno, “Research on the Flammability Characteristics of Aircraft Fuels,” Wright Air Development Center, Wright-Patterson AFB, OH, Technical Report 52-35, Supplement I, 1954, 57 pp. Kuchta, J. M., A. L. Furno, A. Bartkowiak, and G. H. Martindill, “Effect of Pressure and Temperature on Flammability Limits of Chlorinated Combustibles in Oxygen-Nitrogen and Nitrogen Tetroxide-Ni Tet roxide-Nitrogen trogen Atmospheres,” Journal Atmospheres,” Journal of Chemical and Engineering Data, Data , Vol. 13, No. 3, July 1968 (American Chemical Society, Washington, D.C.) p. 421. Noronha, J. A., J. T. Merry, and W.C. Reid, “Deflagration “Deflagration Pressure Pr essure Containment Containment for Vessel Safety Design, Plant/Operations Progress,” Vol. 1, No. 1, American Institute of Chemical Engineers, New York, NY, Jan., 1982. Schuber, G., “Rotary Valves for Explosion Isolation: Approval Without Testing,” European Information Centre for Explosion Protection—International Symposium, Antwerp, Belgium, September, 1989. Zabetakis, M. G., “Flammability Characteristics of Combustible Gases and Vapors,” Bulletin 627, U.S. Bureau of Mines, Pittsburgh, PA, 1965. Zabetakis, M. G., and B. H. Rosen, “Considerations Involved in Handling Kerosine,” Proceedings, Proceedings, API, Vol. 37, Sec. III, 1957, p. 296. F.3 References for Extracts.

The following documents are listed here to provide reference information, including title and edition, for extracts given throughout this standard as indicated by a reference in brackets [ ] following a section or paragraph. These documents are not a part of the requirements of this document unless also listed in Chapter 2 for other reasons. NFPA 68, Guide 68, Guide for Venting of Deflagrations, Deflagrations, 2002 edition. Copyright NFPA

NFPA 86, Standard 86, Standard for Ovens and Furnaces, Furnaces , 1999 edition. NFPA 220, Standard 220, Standard on Types of Building Construction, Construction , 1999 edition. NFPA 654, Prevention 654, Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids, Solids , 2000 editions. F.4 Additional References.

Brenn-und Explosions-Kenngrossen von Stauben, Berufsgenossenschaftliches Institut fur Arbeitssicherheit (BIA) Bergbau-Versuchsstrecke, Institut fur Explosionsschutz und Sprengtechnik, Sonderdruck der sicherheitstechnischen. VDI Richtlinie 3673, Verein Deutscher Ingenieure-Kommission Reinhalting der Luft, Dusseldorf, VDI Verlag GmbH, Dusseldorf, 1979 and 1983. Zabetakis, M.G., Gasfreeing M.G., Gasfreeing of Cargo Tanks, Tanks, Information Circular 7994, U.S. Bureau of Mines, Pittsburgh, PA, 1961.

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Hamyar Energy NFPA 69 - 2002

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